I suspect priority is also being given to maintaining lengthened units on the Weymouths and Cornish branchlines as we are in the peak part of the Summer season.
Don't know about Cornwall, but Weymouth is pretty short. Last Saturday, return trains (second hand data) were:
16:08 - 2 carriages
17:30 - 2 carriages
18:30 - 4 carriages
20:23 - 2 carriages
Large Community Rail party expecting to travel on the 17:30 were asked (on the morning / way down) to return on the 16:08 which they did, and "even that" was pretty darned busy. The 20:23 which used to be an "interesting train" out of Weymouth is now heavily policed and dry and I understand that the 18:30 has become interesting instead.
For comparative purposes, the 18:30 is a summer only train which until 2017 was 8 carriages. The 17:30 has typically been 5 carriages. So capacity is down from 13 to 6 carriages on
GWR▸ trains over than peak return period. However, an additional
SWR» train at 16:54 from Weymouth to Yeovil Pen Mill (part of the Corfe Castle to Waterloo) working does add capacity from Weymouth to Yeovil which traditionally has been the busiest part of the route on a summer Saturday. I don't know how long the SWR train has been, nor how the loadings have been on this "congestion-busting" route section.