Answer:
First, we know that a single liter of milk weighs around 1.032 kg.
Then 7 kilograms of milk is equivalent to (almost) 7 liters of milk.
Now, this is reasonable?
Well, for a typical person, the consumption is between 0.5L and 0.75L
So 7kg of milk may less of what a typical family consumes in a week, but is not an outlier (for example, 30kg of milk or 1kg of milk per week are outliers, which are "nonreasonable") so 7 kg is a reasonable amount of milk.
Now, if the question only refers to the units he used (kg instead of liters) this does not matter, as we know, mass and volume are two different, but equivalent, ways of describing a known matter, as these are related by the density:
Density = mass/volume.
So, for a known material, if you know the mass that you have, you also know the volume.
Then using kg is reasonable in this situation.
