Hello,

I have a big website which is kind of slow because of the amount of articles shown. Is it possible to load more articles when the user scrolls? So that in first instance there only has to be 6 items loaded, and when you scroll the remaining items get loaded.

Yes, You can use an infinite scrolling script like infinity.js or waypoints.js

Does this make the page less heavy in MB's?

It's more or less the same. A few KBs heavier since you are loading a new script.
All the new posts are being loaded on demand so they won't affect loading times.