A RBF replacement transaction can be larger, smaller or have the same size as the transaction it replaces. It basically depends on whether it adds or removes any inputs and outputs:
- If the original transaction has a change output, and the amount in the output is lowered, the replacement transaction will have exactly the same size.
- If the replacement transaction adds another input to fund the larger fee, or adds more inputs than it removes, it will be larger. Note that at least one input must be the same, otherwise the previous transaction won’t be replaced.
- If the replacement transaction adds a large enough input(s) that it can remove more inputs than were added, and/or removes the change output because the input amounts add up nicely, it will be smaller.
I don’t have the data, but I guess that most replacement transactions fall into the first category, and that more transactions add inputs than remove them, making replacement transactions slightly larger on average.
How many bytes does an opt-in RBF signal contain?
Opt-in RBF is signaled via the sequence field that is a part of every input, so the signal doesn’t take any extra space in a transaction.
