It is interesting to note that for much of the 19th century, there were more MPs than at present- the general elections from November/December 1885 to December 1910 saw 670 MPs elected, and the zenith was reached in December 1918 with 707 MPs elected.
With 1918 though, we need to enter a couple of caveats:
The subsequent election, November 1922, was held after the Irish Free State (Agreement) Act 1922, which stated, among other things, that no writ would be issued for an election to the House of Commons from any constituency in Southern Ireland (which became the Irish Free State upon the passing of the Irish Free State Constitution Act 1922 in December 1922) leading to the odd situation that the 1922 general election did not cover all of the United Kingdom, and for 3 weeks Southern Ireland was part of the UK but with no MPs. This brought the House of Commons down to 615 MPs.
It is interesting to look at the Cabinet of 100 years ago, which was comprised of:
So that is 21 Cabinet members- a bit smaller than the present Cabinet, which has 23 (although Sayeeda Warsi, the Minister Without Portfolio, does not draw a Government salary).
What has changed is the increase in the numbers of Ministers of State, Under-Secretaries of State and other junior members of the Government. Then there are the non-Governmental members of the "payroll vote"- the Parliamentary Private Secretaries, who are not members of the Government, but have their foot on the rung below the Government, and are hoping the climb the greasy pole.
One concern about a decrease in the number of MPs is that it increases the proportion of MPs who are on the payroll vote.
Of course, it could be argued that the payroll vote should be zero, and all MPs should aspire to be hard-working constituency MPs sitting on the backbenches. A separation between the Executive and the Legislative would be a radical step beyond anything any mainstream party is suggesting.
One flaw with this is that there would still be preferment- just not from the Prime Minister. The House of Commons would look at ways of holding the Government to account, and logically one way would be through strengthening up Select Committees. As the American system shows, even in a Congressional, rather than a Parliamentary system, there is someone (House Majority Leader, Senate Majority Leader) who decide who gets to sit on committees and who doesn't. There will always be someone you have to keep on the right side of- in one system a Prime Minister who has been elected as party leader by the party membership and who has led their party to an election victory (or near-victory) and in the other system a Leader of the House of Commons elected by that party's MPs.
The only way to keep the proportion of MPs who are on the payroll vote the same is to shed about 10% of ministers. One way could be for there to be a slow reduction as the Parliament goes on- an Under-Secretary of State here and there not being replaced in a reshuffle.
Or there could be whole departments disappearing. Business, Innovation & Skills could be split with training, universities and science going back to Education (& Science), employment relations going to Work & Pensions, trade going to the Foreign & Commonwealth Office or to International Development. Bringing Trasnport back into Environment, Food & Rural Affairs, and/or bringing Energy & Climate Change there
Sort out reducing the size of Whitehall first.