実質: a smaller content amount at the same price
Read 実質的な値上げ when the posted price stays unchanged but less content is obtained for the same money.
An unchanged posted price
実質, read じっしつ, concerns actual substance rather than outward form in this context. 値段は同じでも内容量が減れば実質的な値上げだ。 says that less content at the same price is an effective price increase.
An effective increase here does not require the posted price per package to change. The comparison uses the content obtained at that price.
Include content in the comparison
価格は百円のままですが、内容量は百グラムから八十グラムに減りました。 says that the price remains one hundred yen while the content has fallen from one hundred to eighty grams.
The new package gives less for the same one hundred yen. A higher cost per unit of content and an unchanged package price can both be true.
Identify which item changed
価格も内容量も同じです。 says that both price and content are unchanged. The earlier reasoning based on reduced content cannot simply be applied to that information.
For 実質的な値上げ, identify the basis for the increase: package price, content amount, or the cost of obtaining the same amount.
Does a price of one hundred yen with content falling from one hundred to eighty grams contradict 実質的な値上げ?
Show the answer and why
No. The posted package price stays equal, but an effective increase is assessed from receiving less content for the same money.