First, the tinier the picture the more you have to idealize the shape and make it symbolic, so that the content can still be identified easily. Otherwise you end up with shape and pixel blobs.
Second, the picture you showed is still large enough to make out male attributes. Women have different fat/muscle ratios than men and their legs and bodies look different, no matter how much they exercise and have muscles (unless they take steroids). The legs and joints, especially knee, are proportionally too thick, making it look like thick bones and thick joints. It is entirely disproportionate if compared to a fat woman or a muscular woman. The feet are at least 10% too short measured by the height, the right hand appears to be too big and the left too ambiguous from what you can make out. The pelvic bone should have a few pixels more height. The ribcage is 10% too big. There is no visible female curvature of the spine. The face is too small to make out anything, except for a nose that appears to be too large.
That goes along with the lack of idealization.
Like I said, the differences are minute if you consider the size. But given the size and that it is hard to make out much detail it are the most important factors to determine that it is a dude with breasts.
