Much the same reason as "Why 1/72", if at all possible I want my WW1 models
to be at the same scale so that the entire fleet will all be in proportion
to each other. At 1/5, most single seaters will be about 6 feet in span and
most two seaters about 7'6". The single seaters should all be well under the
7kg limit and with luck the two seaters should be close. So at 1/5 I am aiming
for the largest airframes that will fit under the international limit.
Paul