Unseen tech is more advanced; technology gets smaller. What kind of antenna does your smart phone have now? Remember when cell phones had small antennas? Remember back when mobile phones had large antennas? How many phones have battery packs now? Or removable SIM cards? If I try to open my phone, I've destroyed it. It's a sealed unit. When I get a new phone, I can wirelessly transmit contacts and media from old to new. Consider the TOS data storage card. Looks like a cheap square of thick plastic, no external details. No user interface, no visible antenna, no battery pack, not even a USB port. It's the cell phone version of the floppy disk. When our smart phones look nothing more than a pane of bendable glass, will we say our old flip phones were more advanced because they looked like technology? They had buttons, and moveable parts—that's an indicator of high tech? No.
When I see TOS, I see tech that is higher than TNG, because we don't see discernible technology festooned upon the hull. We don't see hatches, we don't see antennas or sensors, we don't see reaction control thrusters. In and of itself, we must surmise it's technology has advanced well beyond our understanding.
Then came Star Wars and the great greebling, and if you weren't cluttering your hulls, you weren't keeping up with the direction the art was going. When TMP came out, they had to redesign the Enterprise to reflect expectations, and dumbed down the tech in the process. Same with TNG.
The NX-01, no matter what it looks like, we know it's not as advanced as the TOS Constitution class. I think it looks appropriate. What doesn't fit to me, is everything that came after TOS.
