in a nutshell,
the thinner bulbs allow for a more efficient reflector design and closer point of focus, the 4' t8 is 32 watt, the 4' t12 is 40 watt but the 4' t5 is 54 watt, hence the big improvement in the T5. When you combine the improved reflector shape and increase in wattage you can get major gains.
A good relector as opposed to a poor or no reflector fixture is as much as a 300% difference is usuable efficiency.
a T5 with no reflector or a poor reflector design may not anywhere as good as a T12 with a decent reflector
lumens is a measurement of intensity as it appears to humans, it is biased towards the yellow and green spectrums which are least used by plants. A much better value to go by is PAR or even better PUR
Lumens is actually a measurement of the energy plants cannot use efficiently
big lumens and big watts is like a big shot gun, you cant help but get some big energy amounts on target, better yet would be big watts with big PAR/PUR, which would actually be low lumens (relatively speaking)
and keep your CRI around 85, increasing CRI much above that puts more energy into the non usuable wavelengths...
I believe Varun recently experienced this first hand, I sent him a 2'x2 T8 that has a decent reflector, and the results were obvious to him, they produced much more light then his larger t12's with no or poor reflector design, even though the wattage and lumens was much lower
Av
edit:
here is IMHO the single best unbiased/ no BS webpage on the subject, a must read if you really want to learn more about fluorescent plant lighting
the author, Ivan Busko is Director of the Space Telescope Science Institute, Baltimore MD USA