Perhaps she's just the fastest but not necessarily the biggest or the most advanced?
The whole subtext of the show emphasized the idea that the Enterprise
and her sisterships were the biggest and best the Federation had launched. The only time I ever saw that questioned was years after the fact when FJ's technical maual came out and the dreadnaught was a proposed idea. But that's not onscreen so it doesn't really count. And since then there's been a lot of retconning going on.
The JJ films have tended to reinforce the idea that there wasn't anything really special about the Enterprise
and her like, but then again that's a production that has no bearing whatsoever on what was intended in TOS made decades before.