Complete armchair analysis but I'd have thought this is definitely more about empathy than viewing oneself as a thief? I imagine a lot of people would look at a lost $1000 and think "I'd be devastated if I lost that!", and therefore put the effort into returning it. But I have to admit that if I found a wallet that only had $10 in, I wouldn't keep it but I would be a lot more lazy about returning it because "I can't really be arsed and he's probably not missing it much"

In terms of "viewing oneself as a thief", I feel like I would do that even if I kept the $10 wallet, though I guess maybe I would feel like more of one the higher the amount is