Dude, what?
That's specifically the problem: The Shenzhou and Discovery specifically DON'T match that "regular" size we expect from our combat ready multi-purpose vessel.
Of course they do. Shenzhou is similar in size to the Vorcha and Excelsior class starships that serve as a "patrol along the frontier" capacity. It's larger than the Excelsior and Intrepid class ships that do mostly patrol duty in the poorly-explored backwoods relatively close to home. But it's also smaller than the large, heavily armed science vessels like the Discovery or Nebula class, fast general-purpose ships with big guns but a short overall range. And all in all they're much smaller than the mid to long range explorers like the Kelvin Connie, Sovereign, Ambassador and Galaxy class ships.
Furthermore, all your examples are waay off - the NX-01 is certainly not in the same range as the TOS Enterprise
In terms of overall volume, the 1701 is only 7% larger than the NX-01. By comparison: the Refit Enterprise is actually 10% larger than the TOS version. This can be a surprising statistic until you consider the NX-class saucer is much thicker than the Constitution's despite having almost the same width and length, and the bulging catamaran hulls are both large enough to almost make up for the mass of the secondary hull. If NX-01 had the same sized nacelles as the Constitution class, it would be LARGER overall.
And if they had gone through with NX-01's proposed refit -- adding a proper secondary hull Constitution style -- it would be significantly larger than the TOS Enterprise.
The VOY - a smaller exploratory ship - is about the same size as the most prestigious class of starships 100 years earlier.
We never knew what the most prestiguous starships of 100 years earlier actually were. TOS gave us no reason to believe the Constitutions were among them.
But even then, you're still wrong. Voyager's internal volume is more than three times that of the TOS Constitution while being only 15% smaller than Excelsior. It's essentially something of a middle step between them, despite the fact that all three have similar mission profiles.
The Excelsior is a lot bigger than the Connie, and a lot smaller than the Galaxy. It's almost as if.... things had made sense? Like, there has had been a certain size limitations for starships, making our Enterprises always the "biggest" combat ready vessels
Even in the TMP years, that has never been true. Of the 5 starship designs we saw in the TOS era, the actual TOS version of the ship was the smallest of the bunch. Reliant and Grissom (when it was scaled at its original design length) were both significantly larger than the TOS Enterprise.
Meanwhile, 100 years earlier, NX-01 -- which is already similar in size to a Constitution class -- is easily dwarfed by starships of the Vulcan and Andorian fleets. So there have not, in fact, been any binding size limitations on how large ships could be.
There seems, on the contrary, to be an "ideal" size or range of sizes for an ideal mission role. The existence of smalls hips like the Nova class and (modern interpretations of) the Oberth class suggest the smallest and most versatile of these would be "cruisers," ships which are remarkably well armed for their size but also carry the scientific payload you'd usually see on much larger ships. Unlike bigger vessels like Discovery they wouldn't be capable of performing multiple independent science missions at once, but they would be able to collect data from targets of opportunity and investigate specific points of interest. This small category could be called "cruisers" and the TOS Enterprise and later Reliant would be right at the upper limit of that class, hence it is a "heavy cruiser."
But hell - specific starship sizes have always been applied very liberally on Trek before. Until we see a clear on-screen side-by-side view of the DIS and a familiar starship, we don't actually really know how the sizes are meant to relate to each other.
There's an element of wishful thinking in your post, IMO, but it's pretty much a given at this point that Discovery version of Constitution class is going to be significantly larger and more powerful than the Crossfield. They may not be using the Kelvinverse design for it, but it would be amazing if they didn't produce something that was at least in the same general weight class.
