It's partly the way ITV do things, partly because of the facilities they like to use. Their entertainment shows have always looked big and spectacular on screen. Studio 1 at LWT/South Bank/London Studios has always looked very good on screen. It has a terrific two tiered auditorium as well as a large floor space. It's no secret that the BBC have always been a bit jealous of it - as TC8 and even TC1 (
TVC's main 'Entertainment' studios) don't look anywhere near as impressive, especially when semi-filled with seating. There were plans in the early 90s to construct a new Television Theatre at TV Centre that would have rivalled Studio 1 for size and impressiveness but John Birt put paid to that idea and the original plans for TC9 remained on paper.
Have a look at
TV Studio History for a very good description of this project.
Don't ITV actually use two studios next door to each other that can be combined to make the massive space you see on X Factor? Is that Fountain Studios?
ISTR the BBC used Shepperton for the original Fame Academy series - that certainly looked spacious on air - far more so than the recent Celebrity incarnations of the programme where they've been confined to the old GLR building.
I don't think the BBC used to make Parky in TC1 or 8, was it TC4...? But remember that Parky was always produced in 4:3 at the BBC and only moved to 16:9 when it crossed to ITV. That might also give you the impression that that particular show looks bigger at ITV!