I would've thought the opposite. The default subs have large numbers of subscribers (of course), but I feel like they don't have strong communities, or specific themes, that would appeal to an advertiser. Like... /r/funny? /r/creepy? /r/DIY? /r/IAmA? What would compel someone to advertise on those?
On the other hand, there are a good number of non-default subs that have strong themes that I'm certain advertisers would want to tap into. For example, city-specific subs, or /r/bodyweightfitness (105,212 subscribers), or /r/bicycling (126,713 subscribers). These communities are so specific that advertising seems like it would be more effective in those areas than any other. Of course... it needs to be done in a way that isn't intrusive or damaging, and that's probably a bigger challenge in a smaller community.