...Here's another old composite for the fun of it...
That's great - I particularly like the way you've rescaled the crewman next to the shuttle, those crafts were BIG
Well, while it is true that we never saw the forward bulkhead in TOS, Jefferies had intended it to be there and had a specific length of flight deck in mind. It is one of the data points I've collected together in my research.
Granted, my research is based on how Jefferies saw the Enterprise rather than what was seen on screen (in TOS) or listed in canonical sources (all screen references in Trek?), so I don't expect anyone to follow my lead on this... but I thought I'd throw it into the mix...
Clearly at some point the decision was made make the shuttlebay larger than Jefferies had initially allowed for - probably for visual impact.
In fact, shuttles and auxiliary craft have gained greater importance with each new updating of Trek - in TMP the facility is opened up into the massive cargo bay and throughout the movies they're always taking shuttles to board ship. In TNG shuttles are used as private excursion vehicles (despite being limited to sub light!) and in VOY as scouting and survey vessels.
With the way TOS was originally envisioned, a small shuttlebay does work (as Birdog said, it's not your primary mission so devoting a high percentage of your ship to it doesn't make a lot of sense). And with transporters and sensor beams, there a very few occasions where it would be necessary to risk going outside the mothership (another of Jefferies’ concerns). However, the way Trek ended up strongly points to an enlarged shuttlebay facility (if going by on-screen evidence, as opposed to author intent).