The base racial Shipbuilding Rate apples to ships of size 100 (5000 tons). If a ship is a different size, the rate of shipbuilding will be:
Normal shipbuilding rate x (1+(((Class Size / 100) - 1)/2))
This is less complex than it looks. What it means is that the class size is divided by 100 to get a multiple of the base class size of 100. So a class of size 150 (7500 tons) would have a multiple of 1. 5. This multiple is then reduced (or increased if less than 1) to half the distance between the multiple and 1. So for the size 150 ship, the final multiple would be 1. 25.
So if the shipbuilding rate for a size 100 ship was 1000 BP per annum, the annual shipbuilding rate for a size 150 ship (7500 tons) would be 1250, for a 10,000 ton ship it would be 1500 and for a 15,000 ton ship it will be double (2000 BP per annum). A 25,000 ton ship will be built at triple the normal rate and a huge 125,000 ton Star Destroyer will be built at 12. 5x the normal rate. Bear in mind that these ships will still cost the same as before and require the same resources. The shipyard will just use up wealth and resources at a faster rate.
Smaller ships will take longer than the racial shipbuilding rate would suggest. Still assuming a Racial Shipbuilding Rate of 1000 BP, a 2000 ton ship is 0. 4x the size of a 5000 ton ship. Half the difference takes it to 0. 7, so its annual construction rate will be 700 BP. A 1000 ton ship will be built at 600 BP per annum.
The rate at which each ship is being constructed is shown on the list of shipyard tasks and the estimated completion dates reflect the build rates. This enables large ships and very large ships to be constructed in a reasonable time while still taking longer than small ships. Of course you still need to build the shipyards capable of constructing monster ships.
The same general principles apply to shipyard modifications so you can increase the size of large shipyards more quickly. So if you want to add extra capacity to a 10,000 ton shipyard, you can do it 50% faster than adding extra capacity to a 5000 ton shipyard. Adding extra capacity to a 15,000 ton shipyard can be done at twice the rate of a 5000 ton shipyard. This applies to adding capacity, adding slipways or retooling. This means that very large shipyards become possible as their rate of size increase will escalate. Modification costs remain the same, you can just make the modification more quickly
If you look at the last paragraph, Steve says that they are the same in his post.