Glue sizing does strengthen the joint ,because it slows down the capillary action of the end grain. Standard woodworking glues (yellow and white PVA glues) are a combination of water and solids. As the water evaporates, the solids bind to one another. This becomes a problem when the glue is applied to end grain, like on a miter joint. Looked at from the end, a piece of wood is like a bundle of straws. When you put glue on the joint, they immediately begin to pull moisture (and some solids) away from the surface and into the wood. That causes the glue to dry too quickly and leaves behind a smaller amount of solids, resulting in a weaker joint.
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