您好，It's your favourite fruit growing up. 中的 growing up 应该是（现在分词短语）作（后置定语）修饰 fruit。而因为 fruit 和 grow up 属于（主谓）关系，所以用（现在分词 ing）形式。假如把省略的部分加回去可能清楚些：
It's your favourite fruit (that is) growing up.
类似这样的用法还有比如：
The man standing there is my father.
standing there 也是（现在分词短语）作（后置定语）修饰 the man，而 man 和 stand 是（主谓关系）所以用（现在分词）形式。完整句是 The man (who is) standing there is my father.