How can I get uniform font size irrespective of http or https or https www ?

So far I was testing installations of Elgg via Cpanel without thinking much about whether http or www or https. However when I forced https://www the font size overall becomes smaller ! If its just http:// the font size is larger, that is to say, standard. Any idea why this is happeing ? How can I get uniform font size (the larger or standard one) irrespective of http or https or https www ? Have anyone faced this before ? Any solution ? Many thanks.