When a user loads an image using the Image Tab, multiple jpg files are created (Generic, XS, S, M, L, XL). Is there a way to not have K2 create all of these?

This is the default behaviour. It cannot be changed.
Alternatively you can use your wysiwyg editor to upload image but you will some of K2's capabilities.

It's making the size of my site humongous. So I'll have to go in and manually delete the unused jpg's?

I would not recommend this because if you change a setting, your site will be full of 404 errors.