From my own experience, No the returns are not the same, from the first month return to the third it was decreasing.
The addresses that are eligible for SmartRewards are increasing. Yes I think the more SMART you have in the address increases the returns.
You can use this calculator https://smartcash.cc/what-are-smartrewards/#toggle-id-1
This calc says you only get 75 SMART for holding 1000, how did you manage to get 2k SMART then?
Back then there were a few, SmartRewards eligible addresses, so my initial SMART investment was 2000+ the first SmartRewards was I think 945 SMART then decreased now to 587 SMART last time.
I earned it by compounding the rewards and my initial investment, not moving it.
You can use this tool too to see how much you are getting when you start. Also you can display the eligible addresses. Just input the address and you will see how much SMART you are gonna get.
I tried adding my address and my 5000+ SMART will gain me 381 SMART the next payout.
https://smartcash.bitcoiner.me