So I dug up the summary list from the vanilla game code:
"oolite-ship-library-summary-CG" = "Goods container";
"oolite-ship-library-summary-CP" = "Evacuation ship";
"oolite-ship-library-summary-HF" = "Heavy fighter";
"oolite-ship-library-summary-HFr" = "Heavy freighter";
"oolite-ship-library-summary-HT" = "Heavy transport";
"oolite-ship-library-summary-HM" = "Heavy multi-role";
"oolite-ship-library-summary-LF" = "Light fighter";
"oolite-ship-library-summary-LFr" = "Light freighter";
"oolite-ship-library-summary-LM" = "Light multi-role";
"oolite-ship-library-summary-LT" = "Light transport";
"oolite-ship-library-summary-MF" = "Medium fighter";
"oolite-ship-library-summary-MFr" = "Medium freighter";
"oolite-ship-library-summary-MM" = "Medium multi-role";
"oolite-ship-library-summary-MT" = "Medium transport";
"oolite-ship-library-summary-NB" = "Natural body";
"oolite-ship-library-summary-PHF" = "Police heavy fighter";
"oolite-ship-library-summary-PLF" = "Police light fighter";
"oolite-ship-library-summary-RW" = "Recon warship";
"oolite-ship-library-summary-SM" = "Mining installation";
"oolite-ship-library-summary-SA" = "Navigation aid";
"oolite-ship-library-summary-SO" = "Orbital station";
"oolite-ship-library-summary-WMn" = "Timed explosive";
"oolite-ship-library-summary-WMs" = "Fire-and-forget missile";
Does anybody have any idea what the differences are between light : medium : heavy?
Is it mass? Size?
And where are the cut-offs?
Edited to add: see here for an annotated version of this