If you do a Google search or search on the BPs forum regarding forex volume, you’ll get answers with more detail than mine.
I believe that the volume seen on a Forex chart is Tick volume for the currency pair in question. This means it’s describing each price change by tick for the particular broker of the chart being viewed.
Few price changes over the session - small volume bar, many, many price changes over the session - tall volume bar.
Studies have been done concluding tick volume is within a 90% accuracy of actual volume. I’m citing this from a brief Google search done in the past few days. This is not based on an in-depth study of my own.
Of course there are believers and non-believers in the accuracy and efficacy of forex tick volume.