I mean as retrofit for incandescent fixtures. If so large CFL is needed it means that the fixture was for high power and very hot incandescen in the 1st place, so there must be lots of space in it for the adapter
Incandescent fixture could be made to withstand quite high temperatures, so do not have to be as big. Moreover if the reflector surface reflect and the lens pass the IR well, the heat does not accumulate there, so you do not need as strong ventilation, so the fixture so big.
But the CFL emit nearly all it's heat in the form of hot air (so it accumulate not much less heat then the incandescent) and the ballast can not stand this (the tube could be designed for that temperature, it is only matter of tuning the mercury pressure for the target operating temperature).
@Remote phosphor for low pressure MV:
The problem is, then majority of the UV energy emitted by the low pressure Hg discharge (e.g. within fluorescent tubes) can not pass through the air (the O2 is the biggest "blocker"), so it would not reach the remote phosphor. Without this the attainable efficacy would be way less then 1/2 (only the longer wave UV would be "harvested").
And not yet speaking about the burner material, capable to pass this radiation.