That’s really weird, I tried that intuitively a few days ago with an uploaded mesh, and it wasn’t working at all. Now when I try it works with a blank MeshPart in Studio.
Looks like the built-in scale functionality is just sucky, and refuses to scale down the model when one mesh is too small. Takeaway: don’t bother using built-in Roblox Studio tools, scale your meshes in a 3d modeling program to avoid pointless and cumbersome limitations.
Download studio build suite and there is a feature called “scale”. Inserting a number in this section will iterate the selected items’ size. Keep in mind 1 = 100%. So if you want to double the size put in 2. And for half the size, put in 0.5.
Side note, you could just group the mesh by itself and then shift + (resize tool)