Figures are the 2006 mid-year estimates from the Office of National Statistics. The 2001 census numbers are all a bit lower. You'd think they'd have the advantage of accuracy, but are generally accepted to under report. People seem happy to pay their council tax but find filling in the census form once a decade some kind of terrible imposition, I guess. Next census is in 2011, so until the numbers get resynchronised the ONS estimates are the best we've got.
To add a bit of a feel for just how much bigger Birmingham is than Leeds and Leeds is to Glasgow and Sheffield, think Walsall. Birmingham is bigger than Leeds by about the size of Walsall. Don't like Walsall (although why wouldn't you?), think Wolverhampton. Leeds is bigger than Glasgow by the size of Solihull. Bigger than you think that Solihull.
If that's a bit West-Midlands-centric, try this instead. Birmingham is bigger than Leeds by about the population of Plymouth, or Hull, or Derby, or the whole of South Gloucestershire. Leeds is bigger than Glasgow by the size of Rochdale, or Gateshead, or Milton Keynes.
Just to give a final handle on the size of these cities, Birmingham is bigger than Manchester by, well, Manchester. Even the relatively flat part of my little chart disguises some pretty large differences. The additional population Glasgow has over Bristol is about an Oxford or, if you prefer, a Lancaster, a Canterbury, a Norwich, the entire county of Powys. That is, whichever silly way I slice it, a lot of people.