I'm no expert. I did 10x20 or something like that on my shower and didn't back butter. It's been about 5 years and no cracks or problems. Just make sure you are using the cementitious thinset. Not the premixed white one.
I did use 4x12 on my kitchen backsplash with the premixed white thinset, but didn't back butter either.
Actually, I'm in the middle of installing a ton of 12x24 floor tile on an addition, but using that trowel with the huge notches. It yields something like 35 sq ft per bag of mortar! I'm not back buttering. Only back buttering if I need extra material to level one tile to the next. I watched a few videos on this guy on YT. Sal something. He has all kinds of useful information on tile.
I did back butter on floor tile on the restroom, but that was slate and that stuff is so uneven in thickness.