Hmm, I'm not sure that's entirely true. A 100,000 ton destroyer at 7/11 pays 52.5% of its mass for a drive, and 42% for its engine. A 1,000,000 ton battlecruiser at 7/11 also pays 52.5% of its mass for a drive and 42% for its engine. Ships, unlike battlemechs, are flat in their weight costs...