After dipping into the rich pools of много (a lot) and мало (a little), I had the bright idea of looking at what's in between them. So I peered into the Russian middle ground — and discovered that it's pretty sparsely inhabited.
The main Russian adverb for in-between-ness is средне (middling, so-so), which can refer to anything that is in between two extremes, like wealth and poverty, or heat and cold. Unlike много and мало, it doesn't seem to be used as a one-word characterization very often, but you can come across it: Там климат хороший — средне, не холодно, не жарко, а просто тепло (It has a great climate — it's moderate: not cold, not hot, but just warm).
Instead, Russians often spell things out using the adjective средний: Он получает среднюю зарплату (He gets an average salary). Or they use the adverb умеренно (moderately) or adjective умеренный (moderate). Цены в ресторане были весьма умеренные (The prices in the restaurant were quite reasonable).
Or they avoid the boring middle ground and juxtapose extremes. For example, one newspaper article asks, Как установить зарплату сотрудника между "много" и "мало"? (How do you set an employee's salary between "a lot" and "a little"?)
And then they fudge with the phrase нечто среднее (something in between) or где-то в середине or посредине (somewhere in the middle).
Maybe the problem is that in-the-middle-ness isn't a very attractive quality for Russians. Средний and средне can be neutral words when applied to quantities, like temperatures or heights, but when they are applied to other human qualities or output, they can have the sense of mediocre. Here средний isn't midway in-between big and small, it's in- between good and bad. Он средний ученик (He's a mediocre student). Он писал весьма средне, но не настолько уж хуже многих из коллег (His writing was quite average, but it wasn't that much worse than many of his colleagues).
Now this particular middle ground between good and bad is more fertile Russian linguistic territory. You might hear средненько (just so-so), one of those diminutives that seems to be much more damning than the word it was derived from. Актёр играл средненько (The actor was only just fair).
Other words for mediocrity include the adjective посредственный (fair, average) and adverb посредственно (so-so). Сочинения писал посредственно, почерк ужасен, в математике соображал слабо (He wrote fair to middling essays, had terrible handwriting, and a poor grasp of mathematics). There's also заурядный/заурядно (ordinary). То, что кому-то кажется экзотикой, для другого — вполне заурядно (Something that seems exotic to one person might be completely run-of-the-mill for someone else).
Or you might hear a bunch of negatives: как-то не очень (not much; so-so), не так уж всё хорошо (things aren't really so swell), ни хорошо, ни плохо (not good or bad), не ахти как (nothing special, from the exclamation ах! — wow!). Дела у тебя в последнее время не ахти как (Lately your business is nothing to write home about).
I like the phrase средней паршивости (moderately mangy), which can be applied to almost anything: Даже тогда кафе считалось средней паршивости (Even then the cafe was considered just barely acceptable).
Happily, there is one Russian phrase that casts not too big and not too small in a good light. When you're served just enough food or something is affordable or a text is perfectly fine, you can say: Как раз! (Just right!)