Yes, I was not speaking in terms of Etsy, but in terms of Stripe. For Etsy, yes, their cost does definitely increase as transaction cost gets larger, but only because the payment processor they are forced to use (there are no other options) charges a percentage of each transaction. It is on the payment processors side where there is no justification for taking a larger chunk simply because a number is larger.
One of the advantages of digital systems is that per-transaction costs bottom out very quickly. Even if you are doing something complex (which payment processing is not), the actual cost is usually so small as to be very difficult to measure. I work in an environment which, by law, HAS to measure their per-transaction costs in minute detail. They quantify through statistical analysis the mean amount of CPU time, the mean amount of human-intervention time necessary, the amortized cost of support contracts affecting the software and hardware, power usage by the equipment, EVERYTHING. And they are doing something which is many orders of magnitude more complex than payment processing. And the result is that per-transaction the cost is almost negligible. The same service would have cost hundreds or thousands of dollars and taken weeks 20 years ago. Today, there's a guaranteed response time of under 30 seconds for most transactions and they cost less than $10 (with almost all of that price made up of the small fraction of transactions which do require human intervention). These exact same cost-reduction effects of hardware and software growth have affected banking heavily. But none of those advances have resulted in lowered fees, in fact processing costs and transaction fees have only gone up.
I think that it is great that Etsy is moving to accept credit cards directly. Personally I believe the future of our economy (and this might be a 50-year or longer dream mind you) will be moving away from centralized organizations entirely. The benefits companies offered (aggregation of workers geographically, management of distribution chains, aggregation of work itself geographically) no longer exist. The purpose they served can be filled by modern communication technologies and software. For that to even have a chance of happening, however, requires a couple things. Internet access needs to be defined as a public utility or some other steps taken to severely restrict the ability of ISPs to charge higher rates to users which make money via their Internet connection, and payment processing has to evolve to the point where an individual person can accept payment from anywhere for as close to zero cost as possible. If either of those things do not happen, the companies providing the infrastructure service will choke to death any individuals trying to make their living via the Internet. Over the past 30-40 years, thanks to computers and automation technologies, the productivity of a single person has absolutely exploded. The profit most companies make on a per-employee basis is astronomical compared to rates at any other point in history. When that is widely realized, and people recognize that instead of making $35k/yr for an employer they can make $350k/yr as an independent worker (a conservative estimate, it is very difficult to overestimate just how much money is wasted oiling the supremely inefficient machine that is the modern corporation) while working far reduced hours, the need for that infrastructure will be strong.