The EF 70-200mm F/4L IS II has a 72mm filter thread, smaller than the RF 24-105mm f/4's 77mm filter thread, so I would agree that it should be possible to be slightly smaller than the RF 24-105mm, if only by 5mm.
When you really look at the EF 70-200mm F/4L IS II, it's really striking how much of a skinny drainpipe it looks like, in comparison to the extending RF 70-200.
View attachment 193684
Looking back, it's been long overdue that a lens as compact as this gets an external zoom to really make it able to minimize space for those who would be choosing this over the F/2.8 anyway.