Thing you have to realize is that you aren't judging his startup rationally. Nobody judges anything rationally on its logical merits because humans just don't operate that way. Humans make judgments on an emotional basis and then afterward their brain automatically comes up with reasons they had that emotional reaction. So the first thing to recognize is that your opinion of your friend's startup is mainly based on the way he presents it and other emotional factors. Most people will never believe any of this is true since they think human decision-making is primarily rational. It isn't.

Anyway if you have that judgement try to identify the generally non-rational reason you felt that way and tell him. Then you can go through the things like market size, team etc. and talk about them on a rational basis, but with the understanding that investors make decisions on an emotional level also. Probably that reinforces market size being #1 in their evaluation, since a large market means more money.