One important benefit of Smoothy's swap protocol using soft/hard weights and bonding curve is that

Swapping two tokens will only need to perform calculation on two tokens (including penalty), which greatly saves computational cost; and

The gas cost of the swapping will not increase as the token list in the pool grow longer.

By contrast, to compute the number of returned tokens, Curve.fi has to jointly compute the invariant using the percentages data from all tokens. This results in

More gas in data read as the list of tokens in the pool grows; and

More gas in computation as the list of tokens in the pool grows.

The following figure summarizes the comparison of the gas fees: