I wonder if "picking" in the next sentence is "present progressive" or gerund.
If "picking" is gerund, I would like to know the reason.
What's the big idea, picking on a little girl like that?
I'd call it a gerund, since "picking on a little girl like that" is in apposition to "idea" - they are both noun phrases.
Picking ... is the big idea.
The sentence is in the simple present tense.
Some people might analyse it as being "What's the big idea, (to be) picking on a little girl like that" using the continuous form of the phrasal verb "to pick on".