My view of "high volume" is more related to money.

One million stamps is not so much. Only limited editions would not manage that volume.

One million high-end DSLR is way more than one million cheap camera-mobiles.

A company that sells 1k cat-scan equipments over a 10-year period would most definitely see that as high volume, since they would be fighting for market leadership.

A company that sells 1k lamp timers/year would better do something else - their lamp timers are obviously not a market success. And the volume is too small to really press the production costs.

In the end, "high volume" is also a question of risk. Do the volume represents a large percentage of the company income? What would be the implication if 10% (or 1%) of the units is found to be defective and needs to be replaced?

A company that sells 100 different models of shoes don't much care if one of their models suddenly stop selling. A company who have one single item on the market would care alot. So their view on "high volume" would differ, because of the different effects if that volume changes in size.

General Electric don't need to sell many nuclear reactors before they have a "high volume" of sales or the market.

In the end, the word "only" have to be selected based on the original expectations. If the plan was for two million cameras, then one million really is "only" one million. If the plan was for 100k cameras, then "only" would probably instead have been used in sentences like "if only we could have shipped faster".

I have been working with products where 50 installations is huge. And where every factory shipment was many k. So I just can't translate "high volume" into a number. A pilot installation of one product can contain more units than another product is likely to sell in a year.