Ramban: What he meant was that from the beginning (the initial fourteen years) he worked for Lavan with all his strength, and Lavan was blessed due to him - and perhaps Lavan now had a vast amount of sheep - albeit from the weaker animals, and that Lavan's sons' accusations were based purely on jealousy, but were otherwise unfounded.

Ha'amek Davar: I was not lenient even about guarding; if so, surely I did not do overt injustice (like he claims)!