It always works that way, yes. The more I upload (in working days (weekends are a different story)), the more sales I get. It is obviously a matter of quality as well (not really quality, but "buyer suppositions based on thumbnail taste"), but mainly yes, I claim that Shutterstock works that way.
p.s.: It's certainly because of the 25 images per day subscription quota. Buyers check new images under the liked categories and go haywire "downloading everything that breathes"
And consequently they check the portfolios as well. If you don't upload (or don't upload that much), you don't get downloads.
p.p.s.: Other sites (most of them) work completely differently. It is my belief that a big chunk of downloads (again relatively) on Shutterstock happens without targeted search engine inputs.