Why use import mode with Fabric Capacity ?

I just had a ticket where one of our customers ran into trouble refreshing an import mode Model on a F16 workspace. The model size is 400 Mb. Yet I get an error about memory consumption being to high. Roughly 3 GB. So I understand even though the size is small it ca use more memory. The main fact has 20 million rows. And there is no power query transformations. No calculated columns. Yet it suddenly uses 1 GB more than the day before.

I switched it to a Pro workspace and it works fine now.

Why would anyone even want import mode with a Fabric SKU unless the Modell is >1GB?