Volume isn't a particularly relevant metric here. Think of it as the resolution HFT firms trade at rather than a measure of the size of their trading activity.

Volume isn't a particularly relevant metric here. Think of it as the resolution HFT firms trade at rather than a measure of the size of their trading activity. It's not a perfect analogy, but the point is that trade volume is not the best metric for understanding the scope and impact of HFT. It's definitely not an indicator that HFT makes up 30-70% of the finance industry in any meaningful capacity. Here's a thought experiment: how much would things change if HFT firms traded half as much, with double the margin on each trade? Some securities would be a bit less liquid but otherwise almost nothing would change. It certainly wouldn't be a 2x difference from the status quo!