I do find it rather perverse and baffling that Abellio, the same company who run both EMR and Gtr Anglia, deemed that 12 car trains are appropriate for Intercity services services running half hourly between London and Ipswich/Norwich (with an additional third service planned) but that 5 car trains are sufficient for Leicester/Derby/Nottingham/Sheffield services, the latter group all being much larger and far more important places?
I know that traffic flows and volumes, especially on routes to/from London, do not strictly correlate with population, city size, admin boundaries, distance etc (something which has already been done to death many times on this forum) but there is still a very large discrepancy capacity wise between the two routes, especially bearing in mind that Nottingham and Sheffield are large conurbations and important regional centres for their respective regions and Leicester and Derby likewise as large and important sub regional centres, and all of them on a different scale population and catchment area wise compared to Ipswich and Norwich.
Also, Abellio (i.e. Netherlands Railways) would never contemplate, or ever getting away with, running 5 car Intercity trains between Amsterdam and important regional centres.