With "common implementation merging" in the linker, template functions that have the same actual bytecode (because they serialize values of the same size, say,) will all use the same "actual" function under the hood, leading to less bloat. The "pointer and size" function doesn't have any bloat in implementation, but instead there's a little bit of bloat each time you call it, as the size has to be passed as an argument, not hard-coded in the function.
If size REALLY matters, you have to implement it both ways and measure it 