+1, I think the hike in fees is quite high.
Just to defend Bitfinex a bit: 0.15% is still the lowest (correct me if I am wrong) alternative that I know of.
But they seem to be "doing it wrong":
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Doing it right would mean that more customers give less overhead and cheaper prices.
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One explanation, though, would be that this high influx of new users give high one-time investment costs which are all taken at once
Even that is a simplified view of running a business.
The most relevant part your argument misses is to account for all kinds of debt.
Especially when you "bootstrap" a business, this situation is far from the equilibrium. The charges you can take are basically dictated from external circumstances, plus typically you need to attack the competition. Then, as the organisation grows, you start accumulating organisational debts and you start accumulating technical debt. If you want to survive, you're bound to create increased income either/or by expanding into new areas or markets and by rationalisation. This additional revenue typically goes towards the pay of debts.
Additionally, every growth incurs an increase in entropy, which means, the more you grow, the more you have to fight for revenue through rationalisation. A classical example is the difficulty for a startup funder to delegate and "let go" at the point when (s)he can't manage everything alone anymore -- which often results in an outburst of "internal politics" (and even kills a lot of successful start-ups) Thus, personally I would not expect rationalisation gains to be passed on to the customer, with the exception of very old and stable enterprises. Or as a marketing gag.
Unfortunately most people's knowledge about running a business is shaped by the images and misconceptions fabricated by the marketing departments of our industry. Which are clean and slick and not especially related to reality.