Why does J4 do no damage in lane vs Jungle?

I'm still learning the game, but when I jungle J4, I can easily 1 shot squishies, however, I played mid and was ahead in farm and with a full combo, I did only 40-60% of damage? (yes, I fed afterwards). I dont quite understand cause with 1 jungle item of 60 AD, does more than 1 mid item of 55 AD with a passive, does less damage? I feel like the calculations don't add up, and just wondering if I'm missing something?