From what I've seen, many companies see some “big” wins early on, which start to taper off into fewer, often smaller wins the more they test. Typically people are testing things such as moving the form fields above the fold, changing CTA colors/text, updating headlines, etc – smaller tweaks to an existing design, or rather, optimizing for the local maximum. As a result, fewer and smaller wins over time intuitively makes sense, as there is a certain point (a theoretical “perfect” page) where you can no longer improve a design. While it is highly unlikely you will ever reach that “perfect” design, the more you test, the closer you will get. If you are only optimizing for the local maximum, you could be leaving conversions on the table by not trialing “bigger” changes, or rather, optimizing for the global maximum (change to process flow / conversion funnel, introducing a totally new page template, etc). In doing so, it is more likely you will see bigger effects.