1.5x is quite large, I'll illustrate.
It seems too wide for the barrel as is(current BL scale), let alone an artillery launcher will be about 0.6x the width of a tank turret.
__ ________
-----| | | 1x \
-----|__| |___size__/
So if it were 1.5x the regular size, it would be too amazingly huge.
If you want a bigger explosion, then the Explosion radius can be adjusted, as well as damage.