Ill chime in here. Your drain plumbing size will be somewhat dictated by the size of bulkhead the holes in your overflows will accommodate. In my case (dual overflow 125) my bulkheads are 3/4" and 1". Im assuming that's what your tank has.
What I did was adapt to 1" pipe before and after the 3/4" bulkheads and 1 1/4" before and after the 1" bulkheads. I just felt more comfortable this way regardless of the bulkhead being the restriction. If I have a blockage, I know exactly where it is. Not likely since I have my overflows screened off. My dual Herbies run very well this way. Silent. They also start and clear air very quickly, under 30 seconds.
So in short, my siphon drains are 1". My emergencies are 1.25".
Now as to your question about return plumbing being smaller that drain plumbing, no. In fact your return plumbing will likely be quieter with larger plumbing. Unless you go with some gargantuan return pump, the siphons will handle just about anything you can throw at them.
On my system, I plumbed two 1.25" returns over the back. I run a DCT8000 (1600 gph at 100%). With my pump at 100%, my siphons handle all the flow no problem, silently.
Your return pump wont have to work as hard with larger plumbing either (reportedly and issue with DC pumps). I initially setup my system with two 3/4" returns in the "factory" locations in the overflows. The return plumbing was very noisy with a 1200 gpm pump, throttled back or not with ball valves. So were the Dursos. These factors along with the splashing torrent of water in the sump from the drains led me to completely replumb the entire system. I couldn't be happier with it.
