So I've just found something quite interesting about that 90 x 70 x 30 dimension - it was actually in the National Rail Conditions of Carriage. So it seems it's more a case of TOCs sticking to what was there before and not updating it!
2. Condition 47 allows you to take up to 3 items of luggage into the passenger
accommodation of a train. One piece of luggage must be capable of being held in your lap
if required, whilst the other pieces must each not exceed 30 x 70 x 90cm in size.
The NRCoT still appears to have some hangover from this:
Musical Instruments exceeding these dimensions, 30 x 70 x 90 cm
and
Furniture exceeding these dimensions, 30 x 70 x 90 cm
The current NRCoT doesn't actually give any maximum size, I remembered incorrectly, however this page on NRE says:
Items larger than 1 metre in any dimension that you cannot carry without assistance. This includes canoes, hang-gliders, large furniture and large musical instruments
You’re welcome to bring up to 3 items of luggage on the train, at no extra cost. Here’s everything you need to know about luggage on trains.
www.nationalrail.co.uk
What a classic railway mess! Perhaps GBR will take a look at this again - my guess would be them going with the LNER policy which aligns with economy class air travel as others have said, though I do think the 30cm needs revisiting to take into account holdalls and rucksacks if there was to be enforcement (the other dimensions are generous as it is - hardly any mainstream luggage on the market will be bigger than 90x70cm in the longer dimensions). If it's indeed an older policy going back to BR days, it probably harks back to when rucksacks had a metal frame and were typically wider but less deep than the square shape (same size in both width and depth) they normally are now. I did check a few common brands of 75l rucksack (probably the most common size for those camping on foot) out of interest, and found that almost all exceed a depth of 30cm, typically between 32 and 40cm. Of course two of these together easily fit into very little more than the space taken up by one large trolley, as they're normally only about 32-40cm wide as well!
Perhaps "200 linear centimetres" (h+w+d) is the most sensible approach, though that requires people being able to add up!
Of course a rucksack 32cm deep would easily squash into a sizer for 30cm, but increasingly airlines say "if it doesn't drop in with no force needed at all it doesn't fit", which does make things a bit harder with soft-sided luggage of all types, and makes my special IATA cabin size rucksack (precisely 56x45x25cm) a bit of a risk at times!