Well, Nero's incursion in 2230 started I would say a series of changes to Starfleet, with an eye towards combatting what at the time they only knew to be a MASSIVE Romulan ship with superior firepower(compared to Kelvin and its sister ships of the line of that time)not a future-vessel; at any rate, this could well have influenced, and that's how I've been hearing it implied , how and what Fleet builds...altering in the process 1701's design lineage/process, and resulting in a beefed-up, lareger vessel(If, according to the link at wiki I just looked up)it's no faster than the TOS version...still, bigger and better, however you match it up, and to be ready as can be for that hardly-forgotten threat, and more. Fleet got very worried..and necessity is the mother of invention. That, to me, is what it boils down to.Plus. out of universe, they just decided apparently the dimensions closer to the original Jeffries version were not big enough in certain 'environments'...