Probably the efficiency of the gas is the biggest change. Usually the smaller diameter tubes give more lumens per gas volume meaning that less gas and space is used to give the same amount of light. Ballast design also changes, mostly in frequency and startup specs. I don't have much experience with the T10s but it's safe to generalize that watt-per-watt a smaller diameter tube will emit a little more light than a larger (older series) tube. Thinner tubes also present less surface area for light to restrike the tube after bouncing off the reflector so that too is a small advantage.