I only officially dated girls. Not men. Unoffcially thats a different story
Of the girls some are middle eastern. Surinamese and African(Tunisia).
None of them had small bums. Round and bubbly or even ranging to slightly wider and not so bubbly. I think from previous comments, the more perky, the more her bubble but might push directly back.
Not even once have i felt this being a problem. HOWEVER. The stereotype has also influenced girls who might believe this. So i find it to be mental obstacle, and not pysical what so ever.