You don’t need to increase the main chain block size for scaling. I don’t think anyone is proposing that. For bip 300 block size remains the same it just allows for scaling on sidechains. Regarding nodes runners vs miners power dynamic. I am not smart enough to comment. I have read some both arguments saying nodes have full control of the network then I read some counter arguments that miners have more control. Not sure which is correct but I guess it’s nuanced.

1. Miners vs nodes is a great balance of power. Yes, miners ultimately decide what goes into a block but nodes decide what gets broadcasted around the network. If nodes are against some miner collusion they can create a client that counteracts the miner collusion. UASF (use activated soft fork) was one such example during the blocksize war of 2017. "'Even the smallest person can change the course of history." -J.R.R. Tolkien https://bitflyer.com/en-eu/s/glossary/uasf

Nodes matter for different reasons, miners matter for different reasons. Bip300 is a soft fork, meaning backwards compatibility. It does not affect anyone not interested in sidechains. Miners often need to coordinate through softforks to respond to attacks or improvements that would take too long to get community consensus. They do not break bitcoin. They do not add more coins, they do not change difficulty adjustment, and other thinga that make bitcoin, bitcoin. Nodes matter for network affect, verification, ect.

Another thing, since it is miners not nodes confirming withdraws or merge mining a sidechain, this is also why bip300 could be activated without user consensus. If a majority of miners activate it, and users hardfork, they would temporarily be less secure than the miner backed chain. This balance of power is by design. Drivechain is just one person in a mexican standoff on hot day sipping some water and offering it to everyone else holding a gun at eachothers head.