Compare the in ring size differences of Rigo and Donaire and Walters and Donaire, respectively. The size difference would be comically absurd- like Salka-Garcia, if not worse. Walters is a huge 126 pounder.
Although it was noticeable, I didn't think there was an absurd disproportion of size in Salka-Garcia. But you're right. And in fact, Rigo is noticeably smaller than practically every 122 pounder he's fought recently. Let alone a big 126 pounder.
Rigs would be outweighed on fight night by at least 10 pounds. Similar to when he fought Donaire. 10+ pounds is a huge disparity at the lighter weights.
On their respective fight nights, Donaire was bigger than Rigs, by more than Walters was bigger than Donaire.
He probably most likely would ko him. But who knows. Donaire was sort of damaged goods after he fought Rigo. And he moved up in weight so then he'd be even more vulnerable.