I try to get my kids to remember their own things...sometimes they do, sometimes they don't. Everything is always fun and games with Dad, so they forget things all the time, and Dad doesn't remember them, either. Even at home they forget things. As far as clothing goes, I send 2 full sets of clothing, and even this isn't "enough" for Dad - he expects 3 full sets of clothing for his weekends...There is always a full backpack with the boys' clothes, so yes, with everything else they have for school (not much for the kindergartner, but the 2nd grader has more in his bag for his class), it would be a lot. The backpack that is packed for their weekend is a full size backpack. Their school backpacks aren't that big.
ya know, thats still just excuses. As Stealth said you continue to make them responsible for themselves and eventually they get it.
As far as bags go. you pack what is appropriate for the weekend, and thats it. If dad doesn't like it, thats his issue and not yours.
My kid is in third grade and has carried the same ll bean backpack for 3 years, and I could easily fit weekend clothing AND her schoolwork in it.