Well from what I recall I don't think there are any AMD gpus which are 5GB. Nvidia however has a few GPUs which are 5GB however I don't think they are popular with miners. Most are holding the 8GB variants of those GPUs.
So since an ASIC has much higher hashrate than a single GPU, it explains the sudden drop we got in the last few days or so. Makes you question, how many ASICs are actually running behind the ETH blockchain. Given this drop it seems to substantial.
I almost sold out all my 6gb graphic cards because of this but since the original Ethereum is going to PoS it won't be needy anymore and as for the new PoW ETH its probably going to take longer to exceed 6GB memory.