"I have small apples."

Translation: Mam małe jabłka.

[Nik659150]

This sounds like a euphemism

[BrandonSzalony]

Why is this form of Apple feminine while up to this point apples have been considered neutral?

[tadjanow]

It's not feminine. In plural there are only two genders: masculine personal plural and everything else.
This means you can use 'małe' with nouns such as 'psy' (dogs - masculine, but not personal), 'kobiety' (women - feminine), 'dzieci' (children - neuter) etc.

On the other hand you would say 'mali chłopcy' (little boys, masculine personal), 'mali mężczyźni' (little men), 'mali ludzie' (little people), etc.

[mihxal]

"Jabłka" is plural Accusative form of "jabłko". And the noun remains neuter.