you can not have to much return air you can have it to small or in all the wrong places, you must understand the unit suplying the air has a fan that will blow so much cfm. your duct suply is design to move so many tones of air per hour, the fan in your unit onley blows air, it does not suck air, the return air is the amount air that is replaced and since you fan does not suck. the duct work has to be larger than the suply. so it can replace the air that the fan has removed,and the air is naturally replaced, thats why the duct is bigger then the suply, now if you whant to know if you have the correct size duct for your return get a duct calculater and fined out what your tone is divide that by 400 and that will give you your cfm out put and read the directions on your ductulater and you will fined the size return you will need to have for your equipment, over sizing the return would be a wast of monney. use the proper size return, best of luck to you ..