[DS] size restriction on walkers

Can someone explain the reason for the size restrictions on walkers
(only
available in size classes 1, 4, and 5+ according to page 14 of the
rules)?  Is
there some game-balance issue or PSB behind it?  I want to build an
AT-ST,
which would seem to me to be size 2, but I'd at least like to know the
reason
for the rule I'm breaking.
Jed Docherty