Some colleagues I work with who teach Psychology explained this to me awhile back…
There are some people out there who are weight fetishists whose sexual interests fall under some categories of paraphilia. No joke, these people get off on sitting on their partners, or having their partners exert weight on them.
I guess it could be somewhat in the realm of S&M and power play, but there are people who have that kind of interest (smothering, standing on their partners, and other restrictive control games). Different strokes for different folks, I guess.
Maybe she is into that…no harm in asking her for clarification. If you care about her, you should deal with the situation with sensitivity and compassion, as for many paraphiliacs, they take their preferences very seriously and consider it a step closer in the relationship to tell their partners about their wants and desires.