An inch is an inch. Embellishing height seems pretty standard. From 248 in August to 241 now though, I don't see anything remarkable. It was widely reported that he dropped some weight to get quicker before his senior year, so the 255 to 248 change in consistent. Another seven pounds doesn't seem unreasonable. Depending on water, I can fluctuate six or seven pounds in a given day. In Manti's case, we're talking about several months.
As for the size/speed dynamic, I guess it would be nice if he were a little faster. Apparently he's not.