Windows incorrect, Ubuntu truthful (Megabytes/Mebibytes)
From the results of running: man mount : The tmpfs mount options for sizing (size, nr_blocks, and nr_inodes) accept a suffix k, m or g for Ki, Mi, Gi (binary kilo, mega and giga) and can be changed on remount. Binary refers to the base-2 units, not base-10 units, e.g. 1 Ki = 1 KiB = 1,024 bytes.
This a more complete explanation of which file size units do which applications in Ubuntu use see this question: What file size units do applications on Ubuntu use? For more information about Ubuntu units policy see Ubuntu Units Policy wiki.