I'll work up the full detail some time, but the story is this: the Northern Ireland Parliament set up in 1920 had a House of Commons of 52 members, originally elected by proportional representation from the parliamentary constituencies (four from each of the four Belfast seats, QUB and Co. Armagh, five from Co. Derry, seven from Co. Antrim, and eight from Co. Down and the joint Fermanagh-Tyrone constituency, the latter three each also sending two MPs rather than one to Westminster for a total of 13). Bolted on as an afterthought it had a Senate of 26 members, the mayors of Belfast and Derry and the others elected in two tranches of twelve by the members of the House of Commons using STV.
When the parliament was abolished in 1972, the British government proposed instead a unicameral Assembly, with 78 members - so combining the total of the two former chambers - again using the boundaries of the twelve Westminster seats which had just been redrawn (QUB had lost its parliamentary representation in 1950) by proportional representation, thus hoping to broaden the spectrum of those elected. The first election using the new boundaries was thus the June 1973 Assembly election, with most constituencies electing six or seven members (Fermanagh-South Tyrone elected five, South Antrim eight). For the 1982 Assembly election population drift was already such that the numbers had to be rejigged, staying at 78 seats but distributing them differently: South Antrim went up to ten, West Belfast down to four.
The boundaries were already about to be changed due to the increase from 12 to 17 MPs which had been extracted from the previous Labour government by the UUP. I was told the other day that, having come up with 17 more or less evenly sized seats, the boundary commissioners initially proposed keeping the Assembly at 78 members by allocating four members to the smaller seven and five to the larger ten, but in fact the easiest option was to give each seat five Assembly members and that in the end was what was proposed. No elections to a regional body ever took place on those boundaries.
The new boundaries proposed in 1995 further increased the number of Westminster seats to 18, and again the boundary commission felt that, given that the seats they proposed were fairly even in size, it was better to propose that each elect five members than to allocate four to the smallest seats. So we have had an increase from 78 to 90 largely for the mathematical convenience of the boundary commissioners, who share the tendency of their colleagues across the water to try and resolve problems by increasing representation.
The increase from 90 to 108, however, was a political decision. The Forum elections of 1996 (in which I was myself a candidate) were held on a mixed basis - each constituency electing five members by a list system, and then the ten parties with the most votes getting two bonus seats - this basically to ensure that the Progressive Unionist Party and the Ulster Democratic Party would be inside the room. This gave a total of 110 members.
Politicians, not surprisingly, liked the idea of more than 90 members, and a consensus fairly rapidly developed that six seats per constituency, thus 108 altogether, would be satisfactory. The NI-wide top-up was not popular. The PUP and UDP reckoned that they could get in the front door especially with six members being elected from each seat (the PUP were right and the UDP were wrong); for larger parties, the top-up ran against the logic of their organisation in Northern Ireland's locally-based political culture. The exception was the Women's Coalition, who fought to the bitter end for the top-up to be retained, though they too were able to win seats without it as it turned out.
The 1995 boundaries, however, had not been drawn as evenly as previous runs at the problem, and even in 1998 it was pointed out that it would have been more proportional for the four constituencies with the largest electorate (North Antrim, Lagan Valley, Newry/Armagh and South Down) to elect seven members of the Assembly, and for the four smallest (West Tyrone, East Antrim, East Londonderry and Mid Ulster) to elect five, though the final results would have been much the same. The new seats, used in May's Westminster election and (perhaps for the last time?) in next year's Assembly election, have similar problems.
So, the current 108 seats come from 1) giving the Ulster Unionists a Home Rule parliament that they did not particularly want in mid-1920; 2) an afterthought of a second chamber in December 1920; 3) wishful thinking about moderates getting in with larger numbers in 1973; 4) mathematical convenience in 1983; 5) mathematical convenience in 1995; 6) the legacy of 1996's political expediency as it played out in 1998.
The new British government's figure of 600 seats for the whole of the House of Commons is enirely arbitrary, as is the decision to allow the outlying Scottish islands four or five times the level of representation of the country, but at least it is a political decision rather than an accidental consequence of other processes.