"W-Well, he is very big!"

That much she can agree on. Especially since Madoka isn't all that tall, so she understands very well just how big Piccolo is. Neck-crampingly big.

"But, with the way he is.. Don't you think mister Piccolo would rather be called by his name? I'm not sure if he'd enjoy a nickname.."

She may already be imagining the kind of face he'd make if he was called 'big Pi'. In fact-- "Did you ever call him that before?"