First of all, even if you only have a 3/4" line comming into the house, you might have a 1" service line in the yard you can tie into. That's the case for me. 1" PVC from the meter to just outside the house, then transistions to 3/4" copper just before coming into the house.
Second, when it comes to irrigation, larger pipes just mean less resistance and therefore less pressure loss while travelling through the larger pipes.
We're not talking about sausage where you push it through a 3/4" grinder and it stays 3/4" when you move it into a 1" pipe. Water is a fluid operating under pressure. The water goes where ever the pipe will let it. Think of it this way, your hose bibb operates by opening and closing a openning that is about 1/4" is size, but you don't notice that being a problem feeding a 1/2" hose do you?