Yes, Logic IS the tardis ;-))
Basically, Logic's grid is dependent on Logic's tempo.
So a 4 bar loop at 120 bpm when you manually time stretch it (option-drag) can then be made (for example) into an 8 bar loop at 120bpm or a 2 bar loop etc. The tempo stays the same, the loop is stretched plays back slower or faster etc - the region is longer or shorter.
Now, when you enable "follow tempo" on a loop or pre-recorded audio, let's say the tempo is 120bpm and we have a 4 bar loop...
It won't change the length of the loop because Logic's grid reflects the tempo change and the 4 bar loop is still a 4 bar loop no matter what the tempo is changed to. In fact, if the region length changed, then the loop would not be syncing to Logic's tempo or grid.
Hope that makes sense ;-)