My 91 lb male handled heat better than any dog I have ever owned. He could also hunt all day long for 3 or 4 consecutive days. He could have went more but I would have never asked that of him. In the world of hunting dogs, the only size that matters is that of the heart. Guys that have small dogs will claim smaller dogs are better. Guys with big dogs the opposite. I have had every shape and size the last 45 years. My 2 best were my 91 male out of Rik's Risky Raider and an 85 lb male out of Candlewood Nifty Nick. Choose a pedigree and breeder you like and it won't matter if it weighs 50 or a 100 lbs. I am growing quite fond of my 18 month old 65 lb male British lab out of FTW Morayglen Apache.I have a female that goes about 60 lbs , which is still on the smaller side for cbr’s. I’ve had a few including a couple great big ones , one a big Caitlin Jenner type. Most athletic dog of that size I’ve been around.
A small dogs going to handle heat better and have a shorter day to day recovery time.
If not already I’m sure someone out there is going to breed 35-40 lb retrievers