When it comes to tropical divers, you commonly see differences in fin and BCD choices. Some divers choose open-heel, full-size power fins and full-size BCDs. Others choose smaller full-foot fins and a trim BCD. You'll sometimes notice underwater photographers with different-sized fins, too. What's up with this?
Most often, you're seeing the difference between a compleat diver and a tropical-only diver. The former dives in many environments, and therefore has the larger fins and BCD suited to diving with the full-exposure suits, greater weight and higher drag common to cooler climates. The latter always dives with a skin suit, needs little lead and therefore can use an ultra-compact BCD and fins (handy for packing). Similarly, a photographer shooting close-up photos may find compact full-foot fins more than adequate, while one shooting Mr. Whaleshark will need max power to keep up.
But, don't assume that someone with full-foot fins and a trim BCD only dives in the tropics. The most avid compleat divers have more than one BCD and more than one set of fins so they can choose the gear best-suited to the dive.