Learn Croatian: Stolić je pun mrvica. - The little table is full of crumbs.

The diminutive stolić is made by adding -ić to the masculine noun stol (table). When you add -ić to a masculine noun, the new word stays masculine. You can see this because the adjective pun (full) is in its masculine singular form to match it.

Is mrvica also a diminutive here?

Yes, it is the diminutive of the feminine noun mrva (crumb), formed by adding the suffix -ica. Just as -ić is used for masculine nouns, -ica is the standard suffix for making feminine diminutives, and the resulting noun remains feminine.

Why do we use the genitive case for mrvica?

The adjective pun (full) always triggers the genitive case for the thing you are full of. In English you say full of crumbs, and in Croatian that relationship is expressed by putting the noun into the genitive.

If the table is full of multiple crumbs, why does mrvica look exactly like a singular noun?

This is a tricky quirk of feminine nouns ending in -a. The nominative singular is mrvica, but the genitive plural is also mrvica! In spoken Croatian, the vowels in the genitive plural are drawn out longer, but in writing, you have to rely on context and grammar to know it means plural.