Translation:The amount of milk and sugar is small.
This is a wrong translation (improper English). The sentence has a compound subject - two singular items connected by 'and'. Therefore, 'are' should be used instead of 'is'. Please see: https://examples.yourdictionary.com/compound-subject-examples.html.
There is nothing wrong with talking about a 'small' amount of a liquid in this sense. I just did a search and there are many recipes which say to "add a small amount of" some type of liquid. It's just not a specific measurement. For example, "If the dough is very dry and floury, add a small amount of water."
In this case it might be that someone said they are allergic to milk and sugar and the cook tells them that "the amount of milk and sugar [in the item] is small."