Not quite, the "IoT" botnets are particularly small in the great scheme of things. Google "conficker" for example.
Edit: Interesting how this is getting downvoted so much. Conficker had up to 15 million nodes, far bigger than any "IoT" net (when did home routers become IoT anyway?). It's far easier to build such huge windows nets because you get millions of insecure computers with relatively standard hardware and software, not so much with "IoT".
In the past decently sized botnets simply weren't used to send DDoS attacks as much, that's all that's changed.
The original Mirai program tried a little over 60 passwords and it would just brute force into an IoT device.[1]
From what I read, it seems that one specific manufacturer in China is the owner of a lot of devices used in the Mirai botnet attacks.[2]
1: https://github.com/jgamblin/Mirai-Source-Code/blob/master/mi... 2: (I cannot find the link, but it was an article from yesterday)
EDIT:
Found this when googling the strange '7ujMko0admin' password in Mirai: http://www.cam-it.org/index.php?topic=9396.0 So it looks like the Chinese manufacturer that they target is Dahua.
The unfortunate truth is that with the Internet of Things the amount of devices that can easily be taken over has grown so fast that we see DDoS attacks of unprecedented size. Even more unfortunate is that there is no sign whatsoever that this is going down again.