It's because you don't do OTC deals for less than 50-100 BTC at least nowadays.
And people who buy coins worth six figures or dump them are pretty low in numbers, I assume no more than a thousand or two and usually, those are the guys that want to keep a low profile and are not going to brag about it on a social network (unless they are pretty stupid).
But there is one thing I wonder, since the ICO invasion, there are a lot of "companies" and dev groups" that have found themselves with a pile of ETH, probably most of them are also doing OTC deals, maybe the ETH OTC market is even bigger than BTC... not that I care that much but it might be worth for the OP to pursue this path also.
Haven't thought about the ETH dumping. You might be actually right about ETH OTC market being bigger than BTC. Really hard to actually answer that question with legit info. OTC volume is quite a mystery...
If you manage to find out some numbers from different dealers, that would be one interesting topic.
Atm I've just received an 0,2-1% answer. Mostly OTC brokers, just give you ask and bid prices which are 0,5-1% worse than market.