usually when someone says you don't change your bet sizes they mean don't bet pot with the nuts, and 1/2 pot on a draw, or when river betting, don't bet huge with a
bluff and 1/2 as a value bet. If you keep your bets the same it makes it harder for your opponent to get a read on what you are doing. And when you're talking preflop, keeping your bets the same doesnt mean you cant change based on position, It means dont bet 5x the bb on the button with Aces and Kings and then bet 2-3x on the button w/ say suited connectors, or dont bet 2-3x with a good hand and 5-6x with garbage, as you will be to predictable.