SendGrid and MailChimp might be large competitors relative to the size of PostageApp, but the scale difference is orders of magnitude less than the scale difference between WePay and PayPal.

I counted 18 people on the "team" page of SendGrid, and MailChimp's website lists them as having about 70 people working for them. I haven't been able to find exact numbers of employees at PayPal, but I know it's easily into the thousands, and very possibly into the tens of thousands.

I think you have a good point: that relying on being able to move faster than your competitors can be dangerous if your competitors aren't actually slow. I don't think you can really make an accurate comparison between SendGrid/MailChimp and PayPal though. The scale (and culture) is just completely different.