Example :

output of multipath -l
mpath2 (3600c0ff000d8a6bbc03ea34c01000000) dm-2 HP,MSA2324sa
[size=559G][features=1 queue_if_no_path][hwhandler=0][rw]
\_ round-robin 0 [prio=0][active]
\_ 3:0:0:2 sdf 8:80 [active][undef]
\_ round-robin 0 [prio=0][enabled]
\_ 2:0:1:2 sdd 8:48 [active][undef]

Output of fdisk -l
Disque /dev/sdf: 599.7 Go, 599732879360 octets
255 heads, 63 sectors/track, 72913 cylinders
Unités = cylindres de 16065 * 512 = 8225280 octets

Disque /dev/sdd: 599.7 Go, 599732879360 octets
255 heads, 63 sectors/track, 72913 cylinders
Unités = cylindres de 16065 * 512 = 8225280 octets

Difference: 40Go.
Why multipath device have a lower size than lun size ?
How to calculate the rate between multipath device and real device.

If you do an "fdisk -l /dev/dm-2" you should see the multipath device is actually the same size as the disks (sdd and sdf). The way it reports it in the output may have to do with difference in the way multipath and fdisk calculate (e.g. one might be using 1000 and the other 1024).