Considering the NX01, the Kelvin, the TMP and STXI Enterprises all had visible hull plating and weapons ports, I'm of the belief that the TOS Enterprise was some experiment in disguising agressive features in frontline ships that was quickly abandoned.
Of course, NX-01 did have gunports to cover the armament most of the time, and the
Kelvin had at least some retractable guns (although the red-beam guns may also have been retractable, and Robau simply extended them from the very start as a precaution). Kirk's ship could have had the same stuff, merely constructed to somewhat higher standard of finish, and the soft focus of 1960s, sorry, 2260s camera work did the rest...
"In a Mirror, Darkly" gives us neatly retracting phasers on the sister ship
Defiant. The gunports on the
Kelvin need to be bigger because the multi-barrel swiveling guns beneath are bulkier. For all we know, the STXI ship can also go porcelain smooth if she really wishes - or then protrude all sorts of plot devices from unexpected locations if the script so requires.
The cause of that could be something as simple as Dr. Marvick ("Is There in Truth No Beauty") not being involved in this Enterprise's design.
Marvick looked a bit young to have affected the design as such. Probably, much like Leah Brahms, Larry Marvick only influenced the design of the latest standard of engines installed aboard the veteran
Enterprise... So without him, Kirk in TOS might have been flying a compact ship that had nacelles like those of the
Kelvin generation!
To be sure, the jury is still out on how big the STXI ships really are. The hero vessel comes in a variety of sizes, with some surface features favoring the 700m and some the 300m ballpark. The supporting
Kelvin style ships have few or no surface features for establishing scale (comparisons with shuttles or spaced bodies are a bit too ambiguous), so we're left with only one hard (that is, thoroughly consistent) fact: that the
Kelvin style of ships was more or less the same size as the
Enterprise herself, with saucers of similar diameter and engines of similar length.
The next movie will hopefully help us out here. Until then, we need not assume that Kirk in TOS had a ship barely one third the length of his STXI ride - she could have been merely 10-15% smaller.
Timo Saloniemi