I'm sure this has been discussed here somewhere, but my Google-Fu is stumped on how to dig it up.
Has there been any discussion of what happens (if anything) to the Imperium if one uses the small starship design of LBB Book 2 and does not use Highguard at all?
Does the Imperium exists with smaller starships? Is it changed? Does it stay the same, simply with smaller ships?
Small is a relative term when it comes to Book 2 ships. A 5,000 Traveller dTon ship has an approximate volume of 70,000 cubic meters, which is equivalent to a rectangular block of 200 meters long, 20 meters wide, and 17.5 meters high.
By comparison, a World War 2 Liberty Ship had a gross tonnage of 7,191 tons. Gross tonnage is an indication of the ship's internal volume equal to 100 cubic feet per gross ton. That is equal to 2.831 cubic meters, which would make a Liberty equal to approximately 20,350 cubic meters, or about 1,450 Traveller dTons. That means that a 5,000 Traveller dTon ship per Book 2 would have a volume greater than 3 Liberty Ships. A Liberty was rated at carrying 10,000 dead weight tons of cargo. Depending on how you do your cargo volume to cargo mass ratio, that is a lot of cargo for a 5,000 dTon ship.
Now, get ready for a real shock. The RMS Titanic, of iceberg fame, was rated at 46,328 Gross Register Tons, which is equivalent to about 9,370 Traveller dTons. So a 5,000 Traveller dTon ships would be about 1/2 the size of the Titanic. The "Edmund Fitzgerald", of song and shipwreck fame, was rated at 13,632 Gross Register Tons, or about 2,760 Traveller dTons. The "Fitz" was not a small ship by any means. See the dimensions listed in Wikipedia. The "Freedom of the Seas" cruise ship, on which I sailed the maiden voyage, is listed at 154,407 Gross Register Tons, equal to about 31,220 Traveller dTons. That is one big ship, but look at how it rates in Traveller dTons. Barely half the size of the Azhanti High Lightning class
Five Thousand Traveller tons would be about the right volume for a World War 1 super-dreadnought or a World War 2 heavy cruiser in terms of a warship.
In short, the supposedly "small ship" universe of Book 2 provides the player with ships that would be greater in size to all but the largest of the Earth's ocean fleets up to about 1960. Your large ocean passenger liners, World War 2 battleships and battlecruisers, and most aircraft carriers would be the only ships bigger than a 5,000 dTon ship.
I view Book 2 ships are the most realistic for a Traveller universe. Why High Guard came into the picture, I have no idea. I have no use for it.