A fixed fee charges every trade the same share. But a large trade moves the price and costs LPs more. A dynamic fee can reflect that.
Size-Scaled Fee starts at a floor (default 0.3%) and rises linearly to a ceiling (default 1.5%) as the trade approaches a share of in-range liquidity (default 2%). All of it goes to LPs.
Routers that split large orders across pools usually end up paying close to the floor.