I have reread your OP and the one difference I spotted on a second, more careful reading is that the first comparison might have been made in a teasing way, with the intention to provoke insecurity in you, while the second comparison was preceded with reassuring comments that you do look great. Maybe that difference in the intent of the commentary mapped into the difference in your response to it. Maybe you are not entirely correct when you state that you are sensitive to comparisons–perhaps instead, you are sensitive to teasing comments.

Another difference, as mentioned above, is that you are now in college, more mature than you were in high school, and that maturation might have made you impervious to direct comparison.

You mentioned that that other guy is much taller and more muscular than you are. These dimensions of looks are easily quantifiable, but there is more to appearance that is subjective and escapes such direct quantification with a literal yardstick. I imagine that there are girls who find your face more appealing and there are girls who find his face more appealing.

Lastly, I am unclear how you arrived at the use of the term "usually" in the OP's title if you only have two datapoints. And in only on of them, you exhibited sensitivity. One is not enough to say that you are usually sensitive. Or were there more datapoints that you omitted for the sake of brevity?