Achieving horizontal trim is an important part of proper weighting for all scuba divers. When a diver's weights are not distributed correctly, that diver may have difficulty maintaing a horizontal attitude when neutrally buoyant, even if they are wearing the correct amount of weight. Poor trim causes seriously decreased efficiency when moving through the water, which in turn can result in increased air consumption (& correspondingly shorter dive times) and in muscle cramps from having to work harder to swim from point A to point B.
Every diver is different, and many divers complain of having "floaty feet." For voluptuous women divers, however, the opposite trim problem is common; the upper body tilts upwards, and the feet dip downwards, due to more of their body's buoyancy being concentrated in the chest area, and their dive weights being concentrated at the waist or hips. By selecting the correct set of gear, and adjusting the placement of weights and other equipment, busty female divers can improve their trim, resulting in easier & more efficient movement through the water and better dives overall.
Many instructors who are great at finding the correct weight for their students are unfortunately not as skilled in finding the right distribution of the weights to achieve proper trim for their female students with bigger busts. Often, this is because instructors and dive shops do not have a broad enough variety of dive gear available for rental & class use. It can also be due to instructors and DMs being far more used to adjusting trim for the "floaty feet" crowd than they are for the "floaty chest" divers.
There are two ways you can make changes to improve your trim: changing the dive gear itself, and changing the placement and distribution of the gear (especially weights) on your body. Both can be very helpful in achieving proper trim, especially for women divers.
BCDs
For most busty divers, the main adjustment needed is to increase the amount of weight along the upper body, and to decrease the amount of negative buoyancy located at the hips and legs. Selecting the right BCD is important because many of your options for weight placement are determined by the type and model of BCD you are wearing. Choosing a BCD that has pockets for trim weight up by the shoulder blades gives you an easy and convenient way to move some of your weight higher up on your body. Chances are that the larger the cup size you wear, the more weight you will want to have higher up. If your BCD doesn't have trim-weight pockets near the shoulder blades, you can also move weight higher by putting ankle weights around your tank valve, below the regulator. Choosing a BCD with integrated weight pockets as well will also give you more options for spreading your dive weights around, and moving some of it above the hips.
Another option for adjusting your trim with your equipment is to get a Backplate & Wing style of BC, with a metal backplate. Depending on how much weight you need, you can get a lightweight, aluminum backplate or a heavier, steel one. There are many manufacturers who make backplates, and some of them sell backplates that are shorter in length to accommodate divers like me who have short torsos. Additionally, if you don't want to go to a full BP/W set-up, there are also some BCDs available that have a "pocket" designed to hold an optional backplate.
TANKS
One of the best ways to improve weight distribution and trim is to use a steel tank, especially a high pressure steel tank. Steel tanks are more compact than aluminum tanks, and they also remain negatively buoyant throughout your dive. Aluminum tanks become buoyant towards the end of a dive, meaning you have to wear more weight to compensate, and exacerbating any trim problems you may have. (For more about why steel tanks are better than aluminum ones, especially for female scuba divers, you can read my post about it here.)
You can also adjust your trim by adjusting the placement of the tank relative to your BCD. For large-chested women, moving the tank slightly higher in the BCD can be helpful, but the amount it can be moved up is limited by the fact that if you move it too much higher, the regulator ends up right behind your head, causing you to hit your head on it whenever you tilt your head back.
FINS
Switching from negatively buoyant fins to positively buoyant ones can also help to tilt your legs higher and your upper body lower.
OTHER WEIGHT DISTRIBUTION OPTIONS
There are a few more things you can do to help combat "floaty top/sinking feet" trim problems. Don't wear ankle weights on your legs, as they will exacerbate the degree to which your body is tilted. Another item you can use that will give you more flexibility in distributing your weights is a weight harness, in addition to, or instead of, a weight-belt. Weight harnesses can be adjusted to hold the weight higher or lower along your body. For women, I particularly recommend Seasoft Scuba's Seawolf weight harness, because the straps are padded, and curved in such a way that it is more comfortable for women, especially those of us with bigger chests, to wear. For those of us who have a higher body-fat percentage, and who dive in cold water, having all of these options for weight placement makes coping with the larger amount of weight we need less burdensome and more comfortable.
Between switching to a backplate and/or steel tanks, and using shoulder-level trim-weight pockets, a weight-integrated BCD, and/or a weight harness, there are a lot of options for moving different amounts of weight around on your body to adjust your trim. Every diver's body is different, and determining the exact amount of weight needed in different areas or the best way to distribute your dive gear and weight around your body can only be determined by trial and error. Practicing in the pool by changing the portion of your weights that you put in your various weight pockets, harness, or on the tank and seeing how it affects your trim is a good way to get an idea of what works best for you. (Just remember that you will need to wear more weight in salt water than you did in the pool.) Taking a Peak Performance Buoyancy class will also give you a more detailed understanding about how moving your weights around can alter your trim.
Getting your trim just right takes time and practice, regardless of the gear you are using. For women with large chests, this is especially true, since most dive gear (even women's scuba gear) is not designed with our special buoyancy characteristics in mind. But trying out different combinations of equipment and weight distribution is well worth the effort. Once you have found the gear and weight configurations which allow you to maintain horizontal trim, all of your dives will be easier and more comfortable than they were before.
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