Must admit I don't often see the front, but part of the problem is the area round the station has been redeveloped and its original context has been lost. Basically the main building is a nice piece of Victorian railway architecture, the addition to the high level platforms looks like a typical 60's bit of railway building (not architecture). I can't remember what it looked like before.
Most of my journeys, over 60+ years, have been through to Portsmouth Harbour. But it is apparent that Portsmouth and Southsea is one of those stations that has not been "shrunk" well because of declining traffic, Eastbourne is another example.
Searched for the upper platform rebuild - it was 30 years ago.
From
https://www.portsmouth.co.uk/lifestyle/heritage/mourning-the-loss-of-the-old-1-7174832Between 1988 and 1990 there was a complete rebuild of the high-level platforms at Portsmouth and Southsea station. The whole of the old wood and iron roof was demolished and a new one installed.
Looks like the extras when connected to the Victoria elements are looking tired.
Other links with (old) pictures
https://www.aboutmyarea.co.uk/Hampshire/Portsmouth/PO6/Days-Gone-By/Memories-of-Portsmouth-Central/129615-Old-Photos-of-Central-Portsmouthhttp://www.starzina.com/Starzina%20Z%20Railways%20Portsmouth.htmThe "shrinkage" you mention is ironic. Passenger numbers are now at record highs (not sure if Portsmouth and Southsea could be an unusual exception) - but efficiently is so much higher that facilities can be much less, even though traffic is much more.