The smaller the replacement transaction is, the bigger fee rate you have to use for that.
The larger the replacement transaction is, the smaller fee rate you can use for that.
Well, that makes no sense. The fee is in sat/vB so it should adjust with the size of the tx.
In any case, it wasn't like that before.
In the past, when I did an RBF, Electrum would send with the fee I specify. If my unconfirmed tx was at 10 sat/vB, it would only be rejected if I tried to do an RBF with less than 10 sat/vB.
If I submitted the second tx at 10.2 sat/vB, the batched tx would be around 10.2 sat/vB. Not double of the unconfirmed tx.