Warning: pure speculation and conjecture ahead.
All we can be certain about Samson’s physical appearance is that he had long hair. In contemporary artwork, he is normally depicted as someone who would know his way around gym equipment. But what if he was a regular guy—maybe even unimpressive to look at? I’d like to think so.
For one, he was able to fool people regarding how to subdue him (Judges 15:11-17; 16:4-15).
Second, and more fundamental, is that a non-muscular Samson would fit in with God’s pattern of using the things that are not to shame those that are (1 Corinthians 1:27-29). After all, He’s the same God who whittled down Gideon’s army from 32,000 to 300 so that the Israelites would not get fat heads (Judges 7:1-7). Had Samson been a muscular fellow it would be easy for others and himself to attribute his strength to him and not to the God who set him apart before birth.
Of course, this is all fanciful and may well be erroneous. But it’s just a thought 🙂