[wongarsu]

Of course the statement is a bit inaccurate, it should probably say "burning 1 ton of biomass generates roughly 1.65 tons of CO₂"

Oxygen is a good bit heavier than carbon, to the point that about 80% of the mass of CO₂ is oxygen. Since burning fuel is generally about combining carbon and hydrogen in the fuel with atmospheric O₂, producing CO₂ and H₂O respectively, you can get numbers like these even before accounting for high-impact gases like methane.

Edit: and of course photosynthesis is essentially the same process in reverse, taking in CO₂ and energy, adding water for the hydrogen and removing some oxygen (that gets vented to the atmosphere) to get energy-rich biomass.