Why my website is too heavy?

As you can see the attached link, that page is completely empty with just sidebar. But still it’s size is over 1 mb.

And here’s another webpage: https://www.shoutmeloud.com/ which is completely filled up but is still only 800 kb.

So, what is making my webpage so heavy?
Is it the theme I am using?

The page I need help with: [log in to see the link]