Make the saving obvious
Show shoppers how much they save on the size they pick.
Each example below is the actual published output of the Variation sale savings amount widget for a sample product, a Sable brush set: Medium £30 on sale at £24 and Large £45 on sale at £35.10. On a product page the value changes to the size the shopper picks; here it shows the value before a choice or for one variation.
Default text before a choice: the largest saving across the sizes on sale, Save up to £9.90.