old data deletion: "purge now" shows not the advertised db size reduction (Piwik 2.14.0 final) please see attachment
I'm not sure what is exactly shown here (didn't have a look at the code). From what I remember InnoDB does not really free space in the actual data file but think this is not related to this
The estimate is designed to provide an approximate of DB size that will be saved. it's really an approximate estimate, IIRC ie: how many rows will be deleted times the average size of row, plus the estimated index size. it's likely the estimate is not very precise. I'm leaving issue opened but we won't investigate for a while, unless a clear bug is found (ie. estimate completely very wrong). Cheers
Maybe solution is to show what you just wrote on a tooltip?