If you think hiding a saber is hard try hiding a gun...its easier to hide an "I" than an "L".
But you make an interesting point - there is the OT aesthetic of 'saber-as-complex-machine' with greeblies of indeterminate but presumably vital mechanical function hanging off all over to make those sabers look more complex and mysterious. That was a valid approach to saber design in the context of the backstory of the OT [sabers illegal in the Empire, purpose built lightsaber parts hard to come by, kludge-improvised parts commonplace] and still appeals to many fans which is fair enough. There is also the PT aesthetic of 'saber as ELEGANT weapon of then more civilized age' which generally were more streamlined and smaller and more ergonomic befitting an era of numerous well trained Jedi Masters in their prime using subtle and sophisticated duelling techniques with sabers designed for those as opposed to OT where we only saw an aging Jedi Master past his prime with a much-repaired junky saber that almost malfunctioned, a half-machine Sith not exactly a model of dexterity with a saber designed for mechanical hands, and a semi-untrained farmboy full of New Hope with an inherited much repaired saber he didn't even know how to hold correctly at first and later a saber based on his teachers out of sentimentality that wasn't even functionally correct for his lightsaber combat form [Luke used Form V Djem So but his ROTJ saber was based on Bens who optimized his saber for Form III Soresu] which of course Luke didn't know any better because he was so little trained.
Petros saber comes from the PT era when young Jedi build their own saber at an early age but had already been training with practice sabers even earlier in very young childhood as we see in AOTC. Young Petros probably already knew far more about lightsaber combat forms than Luke did even by the end of ROTJ.
So he built a saber that fit his already developing preferences BUT still fit his CHILDS hand size. That makes sense imo since children have less grip strength and would need a smaller diameter hilt to control the 'gyroscopic effect' of lightsabers from twisting it out of their weaker grip. Its ergonomically and functionally correct for a child's saber imo.
I think Justin's size is just about right for Petros saber for the above reasons. Just my $0.02 Cdn...YMMV of course and MTFBWY eh? :)