Washing machine drain size

[JSS]

I have a 1.5" drain pipe for my washing machine that did not have a p trap. I went to add one and read that 2" is the standard now. It is going to be a big job to upgrade that and am wondering is it worth It? I also don't understand why 2" is required when the drain hose itself is much smaller.

 

[Reach4]

See https://terrylove.com/forums/index.php?threads/washer-1-5-standpipe.80530/ #3