If you use the default install mod the space will stay the same, the real files will go in the VirtualInstall folder instead of the Data folder. What you'll see then in the Data folder is not really there.
Then why is Windows reporting that my /Data folder size is increasing when I install texture mods? I thought they were supposed to be virtual? I can see the files in both places, I can see both locations increase in size. If it looks like a duck and sounds like a duck, Windows is going to treat it like a duck. Hence, duplication of data in the /Data folder and the Virtual folder.
This is with a fresh upgrade to 0.61.4 today after not using NMM for years. If that's how it's designed to work, I guess I don't understand the point as mods were working fine when they installed only to the /Data folder. Now, my SSD gets double the writes and mods occupy twice the space.