When creating a new tab in gnome-terminal, using the keyboard shortcut
ctrl+shift+t, sometimes the window is resized to be a bit bigger
vertically, due to the space needed by the row of tabs along the top.
Sometimes, instead of this behaviour, the terminal size (i.e. the height of
the text area) is reduced to make room for the row of tabs.

This behaviour should be consistent. Either it should always resize the
window, or it should always resize the terminal. That, or just always have
the row of tabs along the top, even when there's only one tab, so that
resizing is not necessary.

Reproduce like this:

Open a new terminal.
Add a new tab.
Close the terminal.
Repeat from step 1.

I always see this behaviour when I open a new terminal. I don't think it
happens when you already have a term open. If you have a term, it will have
a 'setting' for this, and will keep doing the same thing. i.e. if it
resizes the window, it will continue to do that behaviour when you bring it
up to a couple tabs, then back down to one, then bring it up again. But if
you open and close terminals and add tabs in them, you should see the
difference in behaviour.