Most players that line up at offensive guard in high school are the least athletic players along the offensive front. This leads to a big misconception on the offensive line the philosophy that if a guy is big, but limited athletically, he can play offensive guard.
When programs evaluate the position, they look for guys with some versatility, the ability to line up on both sides and hopefully enough athleticism to eventually move to center if needed.
A lot of high school programs traditionally utilized the 4-3 defense or "50" front, anchored by two defensive tackles that line up over the offensive guards usually shaded as a "one" technique or "three" technique.
The ability to trap, pull and block angles is very important, but at the college level, these defensive tackles are usually 285-pound or more power guys that are also explosive and quick, and are often good penetrators. To match up physically, programs look for bigger and stronger offensive guards that can anchor at the line of scrimmage, and they might be forced to give up some of the pulling and trapping qualities.
However, the offensive guard must have enough quickness to cut off the defensive tackles and still be able to angle block and short set on pass protection. There are plenty of big, stiff guys playing, but programs are looking for a size guy with some athletic ability. Many kids that are playing offensive tackle in high school usually fit that bill and make the move once they arrive on campus.