It seems to me that adding a measured amount that you know works every time is easier than adding some bit by bit and hoping for the best
Here are some considerations from the great Kenji: https://www.seriouseats.com/the-food-lab-all-american-meatloaf-excerpt-recipe
That was a very interesting article. But, that is far more work than I am likely to ever want to do. My meatloaf is really good. I consider it a fairly easy to throw together dish. I'm sure a lot of people feel similarly about making meatloaf.
Why would someone want to adjust amounts rather than use measured amounts? For one thing, measured amounts of bread can still vary, depending on how fresh the fresh bread crumbs are and if it's dry bread crumbs, they can also vary in dryness. Dry bread crumbs take more effort (or planning ahead, for those of us who make our own bread crumbs) than grabbing a piece of bread that is starting to get stale and tearing it up. The meat might have a slightly different amount of fat or moisture. The eggs could be somewhat larger or smaller. Adjusting the amount by texture at the end is one way to mitigate that. Personally I use crumbled Weatabix instead of bread.