Since very little marshalling occurs at Cambridge and none at Liverpool Street, a 5 car is locked in as such for several hours and even if you don't see it rammed on one leg, it may be on the next, or the one after that, or after that.
There is remarshalling at both Cambridge and Liverpool Street. All of the peak hour weekday 10 car stopping trains involve splits and joins.
2H07 0618 Cambridge North-Liverpool Street splits at Cambridge North after working 1454 from Liverpool Street.
2H09 0638 Cambridge North-Liverpool Street splits at Liverpool Street after working 1010 Cambridge North-Liverpool Street
2H11 0718 Cambridge-Liverpool Street splits at Cambridge North after working 0854 from Liverpool Street.
2H48/2H50 1723/1753 Liverpool Street-Cambridge both join at Liverpool Street.
2H01/2H03/2H05 0448/0518/0545 Cambridge-Liverpool Street all start as single units because of lack of siding space at Cambridge.
I travel on this line all the time. The Cambridge off peak Cambridge services do not need any more capacity than is currently provided.
From my experience of weekday travel I'd agree, but stories I've heard from others suggest that 10 car trains at weekends are needed, and Cambridge does have the trains to deliver that because the units that work the peak hour faster trains are spare.