Let's say someone runs 100 Tor instances on their machine and with their high speed internet connection -> download huge volumes of data through all of those instances.
Are there any mechanisms within Tor to prevent such misuse?
Tor doesn't differentiate between "good" and "bad" traffic. For the network is no difference if there are 100 new users doing their stuff or one person with 100 instances. So if one user decides to do this experiment there is less bandwidth left for other users.
It is slower to download a 50KiB file with 100 concurrent connections (ca. one second for each) than a single 5 MiB file (ca. 10 seconds):
So Tor seems to work better the larger the file is.
You asked about a user who runs 100 downloads of large files. Currently Tor advises a bandwidth of over 100 Gbps and consumes around 60 Gbps. So there are 40 Gbps spare bandwidth. If every download has a bandwidth of more than 400 Mbps it would certainly cause problems. However 40 Gbps cost quite a lot of money. So I guess this can't be done by a single user, at least not for a really long time.