Hi,
I can't test IL2CPP builds directly, but that seems extremely wrong and weird. All DOTween and DOTween Pro build files are a total of just 245kB (inside Unity Editor they have more weight, but only for editor stuff: most of the files are not considered for builds).
What I think is happening, is that Unity creates a blank build (without Unity GUI libraries and probably some UnityEngine stuff) if you don't import anything at all, but importing DOTween "activates" Unity itself, and so it creates a real empty project after that. Can you try recreating a blank project, then creating a blank scene just with some cubes and a couple UI elements, and exporting it again?
Cheers,
Daniele