My wife and I met about 7 1/2 years ago online, she visited me in 2004 and 2006 and I visited her twice in 2008, second time was to get married. For a good few years he job enabled us to chat from the time she got up, to the time I went to bed, (8 hr time difference), most of it in Wap (mobile internet) so there was no chat logs, or way to save all the PMs.
About five months after we got together in 03 we both got computers, which enabled us to share photos, emails, have msn video chats etc.
When it came to the application, I just sent a selection of emails from over the years (I had saved some going right back to 2003), I put together little presentations of our holidays, including a few photos of each time we had been on holiday together, photocopies of hotel receipts, plane tickets, tickets for places we had visited, and a write up of the holiday. I made out a detailed timeline of our relationship, giving significant events.
I would say my application was just under an inch thick, when collected together and easily went into a large envelope.
I think it is quality, not so much quantity when it comes to evidence, and to be honest, if there is too much, it is sometimes hard to see the wood from the trees. I could have easily put a lot more in, but it would have just repeated what had already been shown. I really didn't see the need to give pages and pages of msn logs, which I had, because I honestly didn't think they would have added any further weight to the argument (but would have added considerable weight to the envelope

).
As it happened, we didn't need to have an interview, so they must have been happy with the evidence we gave and didn't question it. I think what you have to think of is that these people are doing this job all day, they can 9 times out of 10 work out if an application is genuine or not quite easily and giving them too much to read could go the other way, because there is no way they could read everything and it could be better to give more solid evidence, but less general chit chat.
That's just my ten cents anyway.