The full install of MikTeX includes loads of add-on packages, most of which you will never use, plus loads of fonts for lots of different alphabets. Rather than install it all, it is better to do a minimal install and allow MikTeX to install required packages and fonts on the fly (assuming you have an internet connection when you are using it).
The portable version of MikTeX (http://miktex.org/portable/) is just under 100Mb which isn't too bad.