In a tradition set down by the beloved Star Trek Enterprise, Discovery has starships that are supposed to predate The Original Series which look like they should be alongside the Enterprise-E fighting a Borg Cube or docked at Deep Space Nine beside the Defiant. The USS Discovery doesn't go quite as far as the NX-01 in retconning earlier starships to be much sleeker than designs we're used to from the time, but it still looks very odd considering the Constitution-class (including the Enterprise) would be in service at the same time.

Hull Composition

The most significant discrepancy between the Enterprise and the Discovery (and the rest of the Starfleet ships we see on the show) is hull composition. The USS Enterprise has a smooth light gray hull, which is only broken by windows and equipment. The Discovery, on the other hand, has a bare-metal shell that looks more like a Constitution-class refit as opposed to the original configuration. Other Federation ships we see in the new series also have that bare-metal look, so what gives?

The difference in material between the Discovery and the Enterprise could be explained away by the fact that we only see two types of Federation starship in The Original Series: The Constitution-class, and with the Remaster of TOS, the Antares-type. Both of these ships have the same, mostly featureless, light gray hull, but we don't see enough of the rest of the fleet in The Original Series to determine if this is the standard finish or if it's reserved for special ships.

The most straightforward conclusion to draw in the absence of data is that for some reason certain ships, including those of the Constitution-class have some special finish applied to their hull for reasons unknown, while others (if not most) don't.

Configuration and Design

This issue is a little easier to bring into perspective. Both the Enterprise and the Discovery share the common Federation starship configuration: a circular primary saucer where the bridge and other command systems are found, a secondary hull where engineering systems are located, and warp nacelles connected to the secondary hull by pylons.

The USS Discovery has a much smaller primary saucer than the Enterprise and much longer and narrower warp nacelles. It also has its primary hull split into two sections, with the outer one rotating when the spore drive is engaged. The USS Discovery's odd configuration, even in comparison to other Federation ships we've seen on the series can be explained relatively easily. It's built around the spore drive.

It's likely that Starfleet modified the Discovery at some point between 2250-2255 to be a testbed for the spore drive technology. We know that after the Discovery's sister ship the USS Glenn was destroyed, the Discovery was the only ship remaining in the fleet with a spore drive. However, the Discovery is a Crossfield-class ship, and in Starfleet, the first vessel of a class always carries the class name.

We haven't heard anything about its destruction, so, there's a USS Crossfield out there somewhere, and it's very likely it looks more like a regular Starfleet vessel. As a newer class, the USS Crossfield might be built with the same aesthetic of the Constitution-Class we're used to, and the rest of the ships we've seen are just older models which were built before the Federations started finishing their ships in the new material. 

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