It was under the Republic that the Romans gained land and generally expanded with the Empire being a time where except for a few wars were essentially focused on consolidating the territories gained under the Republic. At least, this is what I understand - Did the Roman Empire have a grand strategy of sorts?
This older thread discusses the same question, and has answers from u/Tiako and myself.
I quite extensively discuss the back-and-forth debate started by Luttwak that u/bigfridge224 mentions above, though I have only read the original edition of the book.