Could be an imbalance, could be that your racquet could use more weight to it with lead tape. If you're in a car crash what would you rather be in? A mini, or a tow truck. You'll feel the effects more in the lighter mini.
Lead up your racquet boy.
Yes, we know it's all about the technique. But if your racquet collapses every time you volley..you need to add some lead to it...