If Pike was not capable of bringing most of his crew home alive, would he have been allowed to remain in command?
If Pike hadn't brought his ship and crew back intact, would they have let him stay a captain, let alone promoted him to fleet captain? See, this is why the idea that Kirk was somehow unique in bringing his ship and crew back from a 5YM is, frankly, rather idiotic. It's a handy explanation for why, in the conceit with which Roddenberry framed the novel, that captain's adventures would've been chosen over others as the ones to be adapted into an "inaccurately 'larger than life'" TV series, but beyond that it doesn't make a lot of sense and it's overly aggrandizing. I mean, seriously, if Starfleet invested that much of its resources and personnel into multiple deep-space exploration surveys and only one captain came out of it with the ship and crew largely intact, the resultant scandal would be enormous. Plenty of heads would roll at both the command and admiralty levels, and there'd be some massive overhauls of how Starfleet did things. They certainly wouldn't have dared undertake any further "5-year missions" after that. True, TOS did show that nearly every other Connie the Enterprise encountered got destroyed or lost its crew, but extrapolating that to the whole fleet is excessive. And sure, you could argue that the reason Kirk got promoted to Chief of Starfleet Operations was because he was the only marginally competent officer in the whole service and was tasked with rebuilding the organization from the ground up, but is that a remotely desirable interpretation?
Pike's crew was 203 in "The Cage" whereas Kirk had a crew of 430. Maybe Pike set off with the same number of crew that Kirk did later?