@stardepp The checkbox in Settings, Webpages, Use Tab Zoom (Retain zoom level when navigating between websites) controls this behaviour.
If it is
Enabled then you get the behaviour that you describe.
If it is
Disabled then each website remembers its zoom level. I prefer this option. For example, the Serif Forums fit my 1200x1600 pixel monitor @ 110% webpage zoom, but my default is 120%. By disabling this option, each website can be set to suit the view-port or the font sizes on that site.