In Spanish, when you add the suffix ‘itas’ or ‘itos’, it designates something small…. perhaps reduced in size some way. Miguel might have a son everyone would call Miguelito…. Micky we might say in English.
Tunas refers to the cactus fruits…. tunitas would be small, or immature tunas.
There are some species that never really develop the fruit properly to make a nice sweet cactus fruit like so many others do. We have a species like that, we raise it only for the leaves, and break the small fruits off so they don’t suck moisture and nutrients to grow when we don’t get anything from them anyways. But there are ways to pickle or otherwise prepare those fruits… they are really good when picked before bloom opening… and can have a really nice taste.