I have an STM32 onto which I load ELF files in RAM (using OpenOCD and JTAG). So far, I haven't really been paying attention to the size of the ELF files that I load.
Normally, when I compile an ELF file that is too large for my board (my board has 128KB of RAM onto which the executable can be loaded) the linker complains (in the linker script I specify the size of the RAM).
Now that I notice the size of the outputted ELF file, I see that it is 261KB, and yet the linker has not complained!
Why is my ELF file so large, but my linker is fine with it? Is the ELF file on the host loaded exactly on the board?
No -- ELF contains things like relocation records that don't get loaded. It can also contain debug information (typically in DWARF format) that only gets loaded by a debugger.
You might want to use readelf to give you an idea of what one of your ELF files actually contains. You probably don't want to do it all the time, but doing it at least a few times to get some idea of what's there can give a much better idea of what you're dealing with.
readelf is part of the binutils package; chances are pretty decent you already have a copy that came with your other development tools.
If you want to get into even more detail, Googling for something like "ELF Format" should turn up lots of articles. Be aware, however, that ELF is a decidedly non-trivial format. If you decide you want to understand all the details, it'll take quite a bit of time and effort.