Trouble is, First Class is mainly going to be busy on weekdays and only on certain trains on those weekdays. There's not the demand for that much First Class the rest of the time. So whilst a First Class seat may take up twice the space of a "standard" seat but bring in three times the revenue each time it's used, it's really not used anything like as frequently.
FGW have made the best they can with the resources available. I'm sure they'd like to have HST rakes made up with lots of tables for the Cornwall trains busy with families, and ones with lots of First Class for weekday rush hours and another rake with high density seating, but in the real world they've got a good compromise that seems to suit most people. No pleasing all people, obviously
Speaking of first class and money, the weekend upgrade is something like (from paddington)
£5 - Slough, Reading
£10 - Didcot, Oxford
£15 - Swindon, Chippenham, Newbury, Westbury, Taunton
£20 - Tiverton-Plymouth, Bath, Bristol, Newport-Swansea
£25 - Cornish destinations & West Wales
That's quite alot bearing in mind VT is £15 flat for Euston-Glasgow with free Wi-Fi, but then again EC is £25 flat, but you get a hell of a load for your money.
But at least this takes up a bit of first and relieves standard at weekends