ScytheKnight
Wait, what? Yes, cyberdecks were a lot bigger in sr2/sr3; that's the whole point being made. If a bigger cyberdeck could be put in a limb, and an RCC could as well even back then, then your argument that a cyberdeck is now much smaller doesn't work because both decks and RCCs clearly could be implanted in the past, and if anything technology keeps getting smaller and smaller as time progresses.
To recap: this is quite clearly not possible under the rules as written, I don't think anyone is debating this. Whether this is intentional is unknown. House rules have been presented. I would allow it.
To those of you who argue that "it's too big":
First of all, that's what she said. Bazinga!
Seriously, though, a cyberdeck, which is tablet sized, let's not forget, can be made to fit inside of your freaking skull without any diverse effects. I don't think its beyond reason to assume an RCC can be similarly miniaturized and mounted in a cyberlimb or cyber torso. Capacity 6 sounds about right, as you'd no longer need multiple screens, manual controls (joystick, throttle, what have you), because you would get DNI just like an immplanted commlink or deck.
Personal opinion warning:
I believe the writers didn't simply overlook this, but I would still allow it. The numerous refences to implanted commlinks and decks never once mention RCCs. That being said, riggers didn't get a lot of love in SR5, so who knows.