Honestly, anyone who sees Lilith inside her armor is probably just going to assume she's a particularly effeminate looking boy with a taste for absurdly long hair. The Warsmith certainly did.
Only way you'd tell the difference is by catching them outside of it--and even then, only if they're wearing something relatively form-fitting.
In fact. Much as I dislike the direct comparisons, a good representation of what a female Astartes would look like outside of their armor is the
Elemental from Battletech. Minus the bulging veins because the Emperor's biotech is vastly better and doesn't need to look like you're ridiculously roided up--you're huge and muscled but it looks perfectly contained and healthy, rather than "Good god how do you
exist?"
Maia being skinny is because she's about 80% bionics right now, and spent the few years before that wasting away in stasis. Lilith
looks skinny compared to some others because she was a bit on the small side and was borderline on the age limit when they started shoving ceramics and special organs in her so she had less time to grow, but she's still 6'6 and can dead lift close to a ton even outside of her armor.