They change size according to how many you use...
Well, yes, they do to a point. I have both Firefox and Chrome installed my laptop running Vista. Tab width changes on both when the number of tabs open reaches the point where additional tabs can not be fitted into the row of tabs without decreasing the width of all tabs. And, when tabs are closed, one at a time, the tab width is increased until the number of tabs remaining open is small enough that all open tabs can be displayed at standard width.
For example, I have Firefox open with 4 tabs open. If I close one tab, the other three remain the same width. But, if I open a 5th one, then all tabs become slightly more narrow. So, at the native resolution on my laptop screen, 4 tabs is the breaking point where tab width either narrows if more tabs are opened or it remains constant if the number of open tabs drops below 4. I see the same behavior with Chrome.
I believe Opera behaves the same way. So, I don't see any difference between browsers regarding resizing of tabs.