Probably no help but I fly united a couple times a month with mine and have never gotten an overage. Pretty easy to go over weight with all that room. I just use a fish scale to check mine at home before I take off and adjust the load accordingly to stay under the 50lbs.
Size of the bag won’t matter… it’s just the weight. Flown a bunch with southwest and only ever care about weight. And if you pick the agent that looks like they are in a good mood, they won’t nail you with the fee. Always gotta scan the agents for the one in a good mood and throw the charm on