" It means a small portion more. " But, "a little more" is very commonly used with both countable and non countable foods. It has the same meaning as saying, "Would you like some more," but assumes the eater has already had a regular portion, so the assumption is that they just want "a little" more, rather than a full portion.
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syntaxrepairB: I'm glad you like it. would you care for a little more?
If a student asks, why did we use a little when pizza is a countable noun?