Why sometimes the charger for iPad Pro cannot charge for Macbook Pro

Why sometimes the charger for iPad Pro cannot charge for Macbook Pro.

I use the charger for iPad Pro to charge Macbook Pro, sometimes it can be used to charge the Macbook Pro while sometimes it cannot work.

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The chargers have different capacities.

So the ipad charger can charge the pro when it is idle but cannot meet the demand when the pro is working hard.