The phrase "decreasing percentage" means that the amount or value being referred to is getting smaller or reducing over time, compared to its previous amount or value. Full definition
The idea here is to gradually decrease the percentage of carrier dogs via selection down through the generations. (thedogplace.org)
I understand swings like this occur from time to time, but with interest rates moving higher would it be wise to decrease the percentage in bonds to say, 20 %? (moneysense.ca)
A conscious effort was made to increase the carbohydrate content of our foods in order to decrease the percentage fat. (idmprogram.com)