If I recall correctly, the root cause of the problem was not South West Trains having a generous desire to provide more seats for the people of Portsmouth, but being too cheap to pay for the right sort of train for the route.
Originally, SWT had class 442s (with 2+2) seating on the Weymouth line, 444s (with 2+2) on the Portsmouth line, and class 450s and 458s (with 2+3) on the Reading lines.
SWT decided they wanted to standardise, so leased some more 450s for the Reading line, and planned to get rid of the unreliable 458s.
However, they then realised that the 458s were cheaper to lease than the 442s, despite being about 15 years newer. So they kept the uncomfortable 458s and got rid of the comfortable 442s (claiming they were too old and unreliable... they're now working fast trains between Victoria and Brighton for Southern!)
This meant they had to find some 2+2 trains for the Bournemouth line, so they pinched the 444s from the Portsmouth line.
Having the extra 450s now spare, they put them on the Portsmouth line, claiming that it was to increase the number of seats, when it was in fact, just a way of using up their excess of trains with 2+3 seating.
Unfortunately, by the time they realised it was a big mistake, they were unable to unpick all these moves, and instead haves since kept spinning that it's for the good of the passenger.
Having been squished into the middle seat of a 450 four times in the last 2 days, I can sympathise with the commuters of Portsmouth, even though my journey is only 45 minutes!