confirming page size to reduce load time

I'm trying to troubleshoot some speed issues on my site and am having trouble understanding how large my pages actually are.

Using Pingdom, a page is coming up as 15MB, but using Google dev tools, the "transferred" amount is under 1MB. Squarespace recommends both of these tools to check page size in separate areas of their help guides, so I'm not sure which one is a better representation of the actual size. 

Thanks!

Can you share site url? We can check easier.