What is the reason for "blocking" some sites via EOIP?

Question

Mikrotik

I'm completely "green" and I do not understand what could be the matter.
Given:
SITE-1 Mikrotik with white IP 1.1.1.1. - looks into the world of MTU IPOE 1600
SITE-2 Mikrotik with white IP 2.2.2.2. - looks into the world MTU PPOE 1492
Both Mikrotiks are connected by EOIP. Some sites that use large packages do not open on SITE-2, speedtest.net, yandex.ru tbm.ru and a number of others.

Answer 1

Reduce MTU