I have one bigger site in the bingo niche and at this time some smaller sites for casino and poker. I think that I just cannot have one big site, it would be too risky if Google penalizes me for some reason, so I follow Dom’s rule in case of websites: Don’t put all your eggs in one basket.
I want to expand my casino and poker site to be a brandable, big site, but at this time they have very few content items, so I refer them as small ones.
I don’t believe that loads of mini sites can work out too well, too much time to maintain them, but I think it can work for some if that’s your model. (though i refer small sites like those that have 40-50 pages, not those that are just 1-2 page “sites”.)