if you just check the output file, that is not the size that is flashed to the device because it includes debug information and other metadata that isn't necessary to run the program.
one way you can accurately know the size of your app is to program your device with espflash.
if you look in the esp on rust book, they show the output of that tool.
the app/part size is what your program actually uses.
https://docs.esp-rs.org/book/writing-your-own-application/generate-project/esp-template.html?highlight=siz#running-the-code