why did this guy resign?
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No fighting spirit.
White is a piece down, but black has some work to do to sort out his position.
AS Loomis suggests black has a material advantage but needs to sort his position some what, however I would guess a queen side attack would follow the removal of the pawn on c6.. black should maybe look to play nd6, nac4 / ndc4
white maybe has some counter play wit ng6 followed up with moving the bishop and queen into the attack but i think black playing rg8 nullify's that..
So in summary not much for white and black following some manoeuvring can get himself into a good position to win
How about this?
maybe a slight advantage to white.. white probably wants to look at attacking the queen side.. get the rock to b6 threatening bxa6? perhaps double the rooks and threaten b7? or look at a pin on the knight with bd3? I think white can probably go a pawn up attacking the queen side.. make a pass pawn on the c file?
but you never said who resigned?
In the first position, Black can consolidate with 16 ...Na3, 17 Qe2 N3c4. Play might follow 18 Rec1 Nxe5, 19 Bxe5 f6, 20 Bg3 O-O, and White hasn't nearly enough compensation for the piece and pawn.
In the OP Black has a winning advantage, but an escape is not inconceivable for White in that situation; I've pulled it off before.