Kitchen sink line should be 2" by code but your right 1 1/2 pipe should wash itself out better.
I think it comes down to running enough water when using disposal .A lot of older houses had the kitchen
sink drain going out to a dry well separate from every thing else.Its a matter of time before the grease and
starch cause a problem .90% of the time the kitchen sink is the one that's backed up when i get there.
we have no code checkers where I live. House was built in 1923. The bathtub laundry, sinks, and floor drains all go into a seperate leech field seperate from the septic tank. Only thing that goes into the tank is the toilet.