What are the pros and cons of having one big campaign vs a few smaller ones within an ad account?
it doesnt cost much time to manage, but if camp cost go up, your ROI will become tight overall,
if you run several smaller ones you can raise budget on one, probably without hurting the other camps ROI
Sometimes it's not possible to run a "big campaign" from a number of reasons... it might now work on scale, the audience doesn't respond well on scale, costs go up too much when scaling, ads burn out faster ... but you could still stay profitable when running on small scale. This happens all the time with all the traffic sources out there. If that's the case, you can run multiple small campaigns using the same or different creatives and collect a few bucks of profit with each of them. I'm using this strategy all the time.
To be honest, I do prefer to run a limited amount of bigger campaigns over managing 100 of small ones, but sometimes you don't get to choose and you have to stick with what gives profit 
From my experience from affiliate side,
One big campaign(1k daily spend with 1m demo):
Pros.
-Higher revenue quicker, hence higher profit per day.
-Quicker optimization when running with new pixel.
-Lesser management.
-Bigger chance to avoid copycats, as you use your demo up quicker.
-More people react to your ad, Shares/Likes/Comments. Shares are pretty much free high quality traffic. Generally people react more when ad as has already lots of comments/likes.
Cons:
-Way Way smaller ROI, as higher budget GENERALLY means higher cpc's.
-Bad placements, when demo is small(1m).
-Lower CTR.
-When ''targeting'' with text in ad copy, you get just one type of traffic.(Example, product dashcam, Ad says ''DashCams for Cars'', but missing other usage of this product - bikes, home etc)
-Campaign dies quick.
-More bad reports about your ad, when doing affiliate stuff(even when whitehat, people still report).
5 small ads(100usd per one, 1m demo):
-You make more profits out of your campaign.
Do you need more positive things?
Cons:
-Way more work with managing/creating/monitoring.
For conclusion.
I use both. Big campaigns when demo is 100m+. So more revenue could be milked out of the demo before copycats get their hands on it. Eventually take the hit with higher cpc's, but high revenue pays for it and I end up with more profit with lesser time.
Many small campaigns method when I have small demo to work with. This allows me to gain higher ROI but with longer time.
This is not 2010 anymore, back when same ad works in every god damn country after choosing EN language. Today, I assume most affiliates use small campaign approach more.