If they allowed more than 1 shipyard per planet small ships would be really good.
Larger hulls are sufficiently advantaged in potential strategic mobility relative to smaller hulls that I consider it unlikely that you'd be able to get enough smaller hulls to make up the difference. A ship built on a Huge hull, for example, can be as fast as the fastest possible ship built on a Large hull while still having 50% better attack and defense scores than the most powerful possible ship built on a Large hull which uses similar weapon and defense component ratios. A ship built on a Large hull that spends a Small hull's worth of capacity on hyperdrive components has the same strategic speed as a ship built on a Medium hull which spent the same amount of capacity on hyperdrive components, and the Large-hulled ship is about twice as powerful as the Medium-hulled ship, assuming both are built as warships with similar weapon/defense components and component ratios.
Moreover, especially later in the game, the hull manufacturing and especially maintenance costs become increasingly negligible by comparison to total ship manufacturing and maintenance costs, so the cost of a ship built on one hull class relative to the cost of a similar ship built on a different hull class is similar to the capacity ratio of the two hull classes. This would suggest that a Huge-hulled ship costs roughly as much as ten Tiny-hulled ships. A single Huge-hulled ship can have five times the strategic speed of the fastest possible Tiny-hulled ship while still having about five times the attack and defense scores (~= notional power) of the most powerful possible Tiny-hulled ship designed with similar weapon:defense component ratios. If the Tiny-hulled ships being compared to the Huge-hulled ship have exactly half the speed and power of the fastest possible and most powerful possible Tiny-hulled designs, that single Huge-hulled ship offers the same total number of actions per turn and ten times the notional power available per action as all ten of the Tiny-hulled ships you could get for a similar cost (assuming the Tiny-hulled ships are all operating independently; if you begin collecting the Tiny-hulled ships into fleets, the Huge-hulled ship gains more and more of an advantage in actions per turn and will never have less than parity in notional power per action). The Large-hulled ship described in the first paragraph only costs ~33% more than the Medium-hulled ship while having twice the power per action and the same total number of actions per turn.
Arguably, the smaller ships have an advantage in their higher tactical speed, but due to how battle resolution works higher tactical speeds are as often a disadvantage as an advantage. Arguably, the smaller ships have an advantage in the number of opponents that they can target per round, but larger ships can more or less make up the difference with a carrier module or two and often don't need to sacrifice that much for them.
End result is that there really isn't that much going for the smaller hull sizes under current mechanics unless you're in a situation where you don't have the economy to pump out the bigger ships at a reasonable rate (producing additional scouts for initial exploration can be an example, if you're unwilling to rush Cargo-hulled scouts and don't want to use construction or colony ships as scouts; using turn-0 tech, a Cargo-hulled scout costs about as much as three Tiny-hulled scouts to produce and the Tiny-hulled scouts reveal about as many tiles per turn, and depending on the map layout having more slow ships with short sensor ranges can be better than having fewer faster ships with larger sensor ranges, at least for a while).