The 300 GB size of other blockchains refers to publically available data of the size of the Bitcoin blockchain. This information is provided for illustrative purposes only. Source: https://www.blockchain.com/charts/blocks-size
zkApps, or Snarkified Applications, currently exist on a development chain. We expect to integrate zkApps into the mainnet in the near future.
The Mina network enables an uncapped number of block producers via proof-of-stake consensus. There may be factors external to the Mina network that may affect the number of block producers that can access the Mina network (e.g., global internet bandwidth).