So, I recently designed a Sylvan Sorcerer with a Roc Animal Companion, and after spending hours searching through rules and rules forums, I can't seem to identify any clear rules about what space a medium sized rider occupies while riding atop a Huge sized creature (Enlarge Person, share spells).
The rules for mounted combat clearly state the rider occupies the same space as it's mount. This is also obviously intended for a Large sized mount and even makes sense for ground based riding combat.
The muddy area that creeps up in the rules is when your mount becomes Huge sized, even on the ground this seems to play out poorly, suddenly your dagger has 10ft reach and never you mind that you're likely at least 15ft off the ground.
Or a more accurate example for my character, I have a cestus equipped. I'm riding a Huge sized Roc with Flyby attack, and at no point will I be going any lower to the ground then 5ft above my opponent's head. By RAW I can then melee the opponent, with basically my fist, from on TOP of my huge sized mount.
The cestus was a basic example, it's more likely that I would want to deliver a touch spell and fly away with my tail between my legs, but example remains valid. Is there some official rule, or at least some kind of generally accepted rule, be it leftover from 3.5 or some FAQs or forum post that gives a basic idea of how this was intended to be dealt with?
As an aside, I do not need or want help with my character design, I know a lot of folks offer their 2 copper even when not asked. In the same regard, I fully understand the rules for Share Spell/Enlarge Person, don't need a rundown on how an Animal Companion gains Flyby attack, and I FULLY understand that the few rules that are RAW on the subject say you occupy your mount's space. But, it's hard to comprehend taking up the space of a Huge size creature and just gaining reach without any other qualifications than getting on a big mount. Being a massively sized squishy character has a whole slew of negative side effects that can't possibly be intended and I'm hoping to have something substantial to approach my GM with before we meet again.