When copying scale-based sizing between maps, keep all scale stops
Right now if you've customized the scales available in a map so as to add custom scale stops in symbol based sizing, the size settings are not transferred if the layer is copied to a map that does not have the custom stops configured. Eg. my source map has scales 1:64K, 1:128K, and 1:256K with scale-based sizing set at those stops:
If I copy the layer to a different map that doesn't have these stops set, they slide all the way over to the left with no warning:
Once you manually add the scales to the map, the sliders snap back into position.
It would make more sense to me if the stops stayed in the symbology, or warned you that it would copy those scales on paste -- "hey, these features have scale-based sizing at some stops that aren't in this destination map, click OK to copy them over or cancel." It actually doesn't make a whole lot of sense IMO that you have to first add the scales to the map anyway (leads to quite a few extra steps), so perhaps the solution here is to just allow any scale to be used for scale-based symbology.