People upsize their value meals too. And in the airline business, flights get upgauged - put onto a bigger plane - and downgauged - put onto a smaller plane. I have heard uplevel and downlevel as verbs, too. I think the rule, to the extent there is one, is that the core of it should be a verb that means "set the x of". That might mean using an existing verb like vote. But -size, -gauge, and -grade in this construct refer to setting the size, gauge (kinda like size only airline jargon - number of seats - two planes the same overall size can have a different number of seats), or grade of something. The original meaning of those as standalone verbs is to determine the size, gauge, or grade of things. So the noun is getting verbed as part of this process. Once you've performed that particular act of verbing you can then abbreviate "setting your x to higher than it was" into "upX" and "setting your x to lower than it was" into downX. (Upload and download are a whole different thing and don't fit this pattern. They're just jargon. See also sideload.)
Does that mean I can downweight myself if I lose weight? Only ironically. So let's also add that it has to be something done to someone or something else. A teacher could downmark you. A boss could upsalary you. These are nonstandard, but understandable in a way that upwindow or downshoe are not.