For the 60V flex volt battery, which is actually 3x 20v batteries which the tool can configure in series, the advertised capacity in Ah is almost certainly measured at 20V. And 20V is also a marketing term, with nominal voltage being around 18.5. So the 6Ah battery is probably around 111Wh and the 15Ah battery is probably around 278Wh. So only one of them is oversized but you should find a data sheet that clearly shows capacity in Wh.