Consider the following:
You get a match and the map download starts. You have no way of knowing that this map has map wide decals and an enormous file size. You might not even be aware that this concept exists at all. Now you notice that the map download takes much longer than expected. You may now miss your match, maybe you even close the client to "fix" the problem. This can leave you with a broken map, because the half loaded image leaves the map in a broken state. This has already happened and it will happen again in the future.
Because of this I would like to see the file sizes of maps in the matchmaker map pools to stay roughly in the same order of magnitude. The path to have the people manually "upgrade" the map visuals seems worthy to explore further.
Some random thoughts:
We could have a version with and without the decals in the vault. One that is in the matchmaker and one so people can easily install the "pretty" version for custom games and so they can have a look at what they are potentially missing. Not everyone is confident to manually swap files in their folders.
It has already been dicussed why forcing all map downloads is not feasible, I just want to add: It would not only happen once a month, but also everytime you unlock a bigger pool size, which can happen anytime.
Something to encourage people to download the map pool beforehand would indeed be cool. Maybe an element under the map pool button that notifies you that you are missing maps and gives you the option to download them all with a single click. Maybe I'll look into it once I finished the matchmaker and league projects.
Lastly: I really like the visuals of your maps and how you are pushing the boundaries. Keep them coming!