Massively is a massive overstatement on these services. And TBH most services these days. First Class usage has dropped considerably - most firms now won't pay First Class for business travel. In any case, except for Intercity, First is around 1.5 to 1.6 times Standard. Given the % of passengers that travel first class (Southern is 2% for example), any subsidy would be minor.
What is true is that much of the First Class travel is full price, and that it's quite full when the trains are at their busiest. When they're empty there are generally plenty of seats on the train. So there's generally more revenue to be earnt from keeping a first class offer.
Some trains could do with a reduction in their first class capacity, but that is expensive and would generally only be done as part of a general refresh.
It actually increased under GNER, which I was quite grateful for, because I sometimes got to travel on a First Advance which was cheaper than an SSR with a railcard. Weekend First has also been quite useful in unlocking capacity for a small suppliment.
In terms of class and standard, look at what is labelled as what. Basically, consider all the former Intercity routes, plus Waterloo-Weymouth/Exeter, Kent Coast (Javelin), Paddington-Worcester-Hereford, Trans-Pennine, XC Turbostar-worked, Liverpool-Norwich, Scotrail Express and former Alphaline, those would be Intercity. Intercity means guaranteed first available (proportion decided by route), guaranteed catering (restaurant/buffet/trolley depending on route), low-density seating, Off-peak and Super-off-peak tickets available and big luggage facilities - basically all the comforts of an express passenger train, being precisely what it is. Commuter expresses around London (Kent Cost non-Javelin, Brighton Main Line, Portsmouth Direct, airport expresses, Cambridge Cruiser, Oxford Flyer, etc) might not need the catering or luggage space, and might require a higher seating density, but on the whole they are basically the same - I'd apply this to any non-Intercity Class 1 train. Class 2 trains can be divided roughly into outer suburban, inner suburban/metro, rural medium-distance and rural stoppers. Depending on route and service pattern, seating density can be mid to high, first might or might not be available, same for catering and luggage space. Inner suburban trains would be designed to optimise the number of people on-board, the others more of a compromise, since people might spend more time on them (several hours on some medium-distance trains).
Sound familiar? It's basically what BR did before sectorisation. Apart from modifying the position of some non-Intercity Class 1 trains (downgrading by NSE and RR IMO), it's what they did after sectorisation as well.
