Aliens are subject to the laws of nature too, and eventually they discover advanced materials like nylon, and engineering concepts like the cantilever span and dynamic tension, leading inevitably to the invention of the brassiere. Increasing brassiere capacity creates a demand for more mammary tissue to fill it, which leads inevitably to the invention of silicone implants and similar cosmetic surgery. Further advancements, such as the Federation's discovery and use of antigravity, create further demand for mammary tissue...ad infinitum.

Actually, by the standards of the 23rd century Seven-of-Nine is considered flat-chested. The selection process for StarFleet personnel is similar to the height and weight constraints on present-day astronauts, and takes into account occupational hazards such as explosive decompression ("she's gonna blow, Captain!") and the need to fit into standardized safety harnesses and uniforms. Very important is the ability to fit through standard automated doorways within the time constraints of the sliding panel (ouch!). You would be seriously impressed by the equipage of planet-bound Federation women and their anti-gravity brassieres!

You're welcome. Glad I could shed some light on this important subject, one so near and dear to our hearts.

--Robert