Applications on Ubuntu use different file size units. Binary units are multiples of 1024, whereas decimal units are multiples of 1000.
Which applications on Ubuntu use which system?
Before reading this answer, make sure you understand what the different units and the different systems are. This is a quick reminder.
The IEC units are 1KiB (1024 bytes), 1MiB, 1GiB and so on. The SI units are 1kB (1000 bytes), 1MB, 1GB and so on. Ubuntu's unit policy mandates that IEC and SI units be used only.
The traditional units are 1KB (1024 bytes), 1MB, 1 GB and so on. Ubuntu's unit policy only allows using them for backwards compatability.
I'm making this answer a community wiki, so please add applications to the list and keep it up-to-date!