It's not certain the big lummox is a cockerel, but yes, his relative size may be one predictor. Look at his budding comb and wattles. It's highly unusual for a pullet to develop those at such a young age, while cockerels wattles do tend to emerge early on. How do his feet and legs appear? Are they thicker and longer relative to the others? They appear so to me. If he starts crowing soon, you'll know for sure. I recently had a cockerel who crowed at six weeks.
In judging the age of chicks you aren't certain of, compare their size to ones that you are certain of. If you got your chicks when they were just days old, you can expect a chick who is one week older to be around double the size, and each week thereafter, doubling in size again. You have the chicks in the flesh while we just have two-dimensional photos. You can better assess the age by measuring this little "godzilla". I maintain the reason he's so much larger is due to him being older. He's still a little cutie, though.