Why is Latex so big?

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 downloadable version of MikTeX (https://miktex.org/download) is just about 240Mb which isn't too bad.

PS: The portable version is exactly the same as a standard installer, just the instal procedures are different.

The portable version http://miktex.org/portable/about amounts to 100Mb (compressed) I guess that it's enough for typical use.