Economies of scale are quite self-explanatory: the bigger your production (or trade in someone else's products or services), the more you can profit from reusing the same equipment or using its capacity more fully. In economies of scale, repetition is the mother of profit.
This is also how translation agencies work. They put their project managers to maximum use and beyond. They usually work with a greater number of translators, and a greater number of clients. The scale of their operations allows them to hire support staff as well, to free up translator time and PM time. It also enables them to follow their seemingly favourite strategy, i.e. cost leadership, meaning lower prices for end clients, which, in their case, might even actually be more profitable than investing in marketing to claim higher rates. You can't really follow the same model because you're limited by the amount of content your one brain and two hands can produce or convert.
From a different perspective, you can't possibly profit from translating (or illustrating or even importing) a single book for a single client for the typical price a translated book at a bookshop. Since so many copies of the same book are sold in bookshops across the country or worldwide, the price of one book can be so much more accessible. This is how reproduction or serial or mass-scale production works.
With the exception of royalties for literary translators or negotiated fees for high-octane marketing translators working on short texts of which the critical importance is much greater the length, translation typically does not cost less or more depending on the frequency or intensity of its intended use (for example the number of copies or displays or some other instances of use). Our traditional remuneration scheme is simply different and does not account for this particular parameter.
So, besides the fact that you're made to suffer economically because agencies can't understand or don't care that you won't profit from more words at a lower price per word, are you also clocked out from any positive effects of scale?
Nope, you aren't.
It may not affect you directly, because your own production scale is one brain, two hands, one keyboard etc., but it certainly affects that part of the client's business in which your translation is going to be used.
For example that manual or leaflet you translated for a manufacturer will not be used by only a handful of people the same way the translation of a birth certificate, marriage licence or school diploma would be used in a single administrative procedure and forever kept on file, unused.
On the contrary, probably quite a lot of people will eventually end up reading it and perhaps relying on it to produce or sell something, or litigate it or whatever. The same applies to anything you translate which is ever going to be published anywhere. Whatever the audience is, it is most likely going to be larger than the guy who sought you ought, his boss who okayed the project and the accountant who paid your invoice.
Similarly a contract is probably going to be read by some lawyers, accountants, consultants, sales people, a judge or two perhaps, perhaps not a huge crowd of people in all, but it will quite possibly affect the operations and thus the profits of an entire production plant or transport company or design studio or whatever else it is.
And business translation still costs mostly the same rates in the broad market as community translation and translation of civil records for individuals or only a small fraction more.
Hence, rather than dropping your rates in response to price pressure from the largest business clients, you should reminding them and explaining how they're going to use your translation again and again and again. It's not like they don't already know. Instead, chances are they simply don't see the connection or don't see it as a good reason to pay rates more in proportion to the scale and importance of the project just like they'd probably prefer your skill level and attention to detail to be.
Remember: Managers, consultants, lawyers, designers, engineers, analysts and similar people are paid higher wages than the bottom of the scale because their work leverages the work of many other people, either directly or by being applicable to the product of it or the process. The thing is, the translator's work also does that.
It does involve a lot of typing, sure, but so does the work of the other professionals. And yet a lawyer hammering a brief or an engineer wrapping up the results of a study or test isn't really seen as just typing. Neither should be the translator who translates all of those people's mental operations, but we do have a bit of an image problem. Hence you may need to do some explaining and some persuading.