The fact is, none of us can even prove that Abraham Lincoln ever existed at all, much less Jesus. When it comes to historical belief, we are all at the mercy of written accounts, that could for all intents in purposes, have been complete fiction. Yes, some historical figures are more defined than others, in that there are many accounts of them and their actions, as well as written accounts that can be tested for age. However, as the individual, we must all take these "facts" on the authority of another for we cannot be there to witness their reality in person.
As for Jesus, as described in the NT, he was an ascetic. As such, he was likely dangerously skinny by the time he was preaching and prophesizing. He gave up his carpentry trade sometime in his early 30s to preach the gospel. In so doing, he gave up all his wordly posessions and lived in immense frugality, likely not eating much at all. If you look at pictures of other ascetics, they are remarkably thin and weak. Jesus might have been strong in build at one point in time, but by the time he was crucified he was most likely a very weak and frail person due to his lifestyle choices.