First and foremost, yes, bigger fish produce more eggs and potentially have better genetics. Releasing these fish is EXCELLENT for most fisheries.
Aside from this, I’d argue there are a lot of misconceptions out there about keeping and eating larger fish or river fish.
While some may be true, like the potential for more contaminants, filleting out the center line removes most of them and its a practice I use and recommend on all walleyes regardless of size.
The other thing is ensuring there is absolutely no hint of blood or fish slime anywhere near your meat. I am 100% convinced that THIS is the #1 reason your fish will taste bad, and the blood and slime from different lakes/rivers can effect your meat in different ways.
I’ve eaten quite a few fish from the Hastings portion of the MS river, the dirtiest river water you can find around here, and the meat is sweet, succulent, firm, and better than fish out of any lake I’ve tried.
I feel the slime and blood on a river fish filet is critical to thoroughly rinse off. In my experiences, the effects of slime/blood on the meat with lake fish is more forgiving to sloppy fish cleaning. The end result for the average Joe hacking up a walleye filet – lake fish taste better.
Another thing I don’t think many people consider with larger fish is that thicker flakes of meat make for a much more pleasurable fish-eating experience. I’ll take 1/4″ flakes over 2 millimeter micro-flakes any day.
Here’s a little blog post I wrote up earlier in the year on how I like to fillet fish: http://burchfishing.blogspot.com/2012/03/tips-on-fish-preperation.html