Back in the 90's I remember that I was always fascinated by the very compact ballast compartment of these sylvania CFLs. Yesterday I've found this EOL sample and I've decided to dismount it to see how it was engineered.
To my surprise I've found that not only the ballast is very small, but it has a sort of fast start system that increase the lamp current until the lamp warm up:
The ballast coil can regulate it's inductance by a moving airgap, the small ferrite cylinder is out of the inductor magnetic circuit when the lamp is cold and them when the lamp warm up it's pushed into the inductor by the bimetallic coil.
That's really a good idea and impressive engineered in a very small place.
The PTC resistor for warm start and some capacitor are placed direcly under the tube and the rest of the ballast is into the E27 cap (and full of some sort of resin).