I've just been advising an academic on applying to the ESRC. There used to be a substantial difference between the success rates for their standard and small grants. This has now pretty much disappeared, so I suggested that he shouldn't worry too much about trying to scrape it in under £100k upper limit for small grants.
Of course, value for money is a consideration, but more important is that the costs are justified, necessary and realistic.
Other than that, I suggested that he bear in mind the process that the application will go through, and who will be assessing it. Small grants only get seen by two members of a 'Virtual College', with the final decision being made by a member of the review panel; standard grants get seen by the whole panel. Details of Virtual College members are here; Panel members are here; and what the different panels cover is here.
From these it's possible to get a good idea of who will be asked to review the application. Is it someone who would be sympathetic to your work? Or is there a danger, if you put in for a small grant, that it will be shot down by someone who's unsympathetic? Would it be better to get a full hearing from the whole panel, including members who might know little about your area?
So construct your application according to the needs of your project, but always bear in mind your audience.