A few things about the Manning comp:
1) Manning was a drop back passer in a pro style offense at UT. He was not called on to run as a substantive part of the offense. Heupel's offense, on the other hand, works best when the QB runs and running is part of Nico's game. The need for Nico to sustain hits will be greater than the need was for Manning to sustain hits.
2) Manning, as advanced as he was for a freshman, still wasn't going to start as a true freshman, but for some unlikely events. He hoped to redshirt, in fact. It took injuries to Colquitt and Helton and then beating out Stewart in order for Manning to start.
3) We went 8-4 in Manning's freshman year, in large part due to the fact that we were limited at QB. That was a down year at that time. And that's with top of the SEC level of talent on our roster. PM had a great Oline in front of him and future NFL starting RBs behind him, along with a pretty good defense. We're certainly hoping to do better than 8-4 next year.
I'm not saying Nico can't or won't be the starting QB and do well next season. He may. I'm just saying that the Manning comps aren't necessarily indicative that he'll be successful or that being too skinny shouldn't be a concern.