The phrase “other fish to fry” goes back to the 17th century:
According to a sloppy Google Books search, variations appear with these frequencies:
It seems to me that “larger fish to fry” ought to mean the same as “bigger fish to fry”, but it sounds weird. Is that just because it's unusual? Or is there some pattern to the way English uses ‘bigger’ and ‘larger’ from which one could predict that ‘bigger’ would predominate here?
A (Dutch) co-worker informs me that the Dutch version of this phrase is “andere katten te geselen”. Instead of frying fish, they are flogging cats. Who knew that the Dutch were so depraved?
(Not really related: OED Quick Search for “other fish” asks: “Did you mean: motherish?”.)