What i have seen of contract staff, especially offshored or imported from Abroad to the UK is that you may get a lot of cheap manpower, but the quality of the work is far poorer that if it had been done in-house.
They normally deliver "exactly" what you have asked, Unles you give them a specification that details every last thing that they need to do and you specify exactly how it is to be written then there is no guarantee that you are not going to have to re-write the whole lot.
Just throwing more bodies at a problem is a wast of time if the poeple doing the work have no buy in to the project and no experience of what you want doing.
When i have worked with experienced internal staff you can usually ommit a lot of information in specifications because they already know how you work and how the systems do things. IF there is something that they do not understand or something that does not look right then I would expect them to ask questions. Offshore if you spec it wrong, even blatetly wrong they will probably never question it and just code it to spec.
If you have to specify every dammed thing, and coding can be very complicated (ask any coder) then that type of spec will require at least three to four times as much time to write.