is the 2560 flash not actually 256 KBytes? avrdude writes/reads 261258 bytes

[Britton Kerin]

avrdude -c avrispmkII -p m2560 -P usb -U flash:r:downloaded_image:r

produces a file 261258 byts long.

When writing the flash I get a message:

avrdude: 261258 bytes of flash written

I expected this number to be 262144.

Is the 2560 flash memory actually slightly smaller than the start of
the datasheet states, or is avrdude doing something strange?

Britton