You are talking about SO2 amount per cubic meter. This is how this gas content is expressed at ground level.
What Windy shows is totally different. It is the amount of SO2 in a column from ground to the top of atmosphere expressed per square meter.
Click the small circled i in bottom right of your screen to get information about this layer.
This SO2 layer shows the plume of the volcano in altitude, not the SO2 content at ground level which is given by the Air quality stations in Windy. For example:
At this Air quality station with a 89 quality index, the SO2 content is 11μg/m3 at station level while in the total height of the atmosphere the value is 209.35mg/m2.