Summary output not reaching a minimum of words

The trouble I am facing with several articles or documents is the minimum words. I’ve read similar experiences in this forum.

If my originial documents contains 20k words, and I ask chatgpt (plus of course) to summarize in 1000, the output barely reaches 600. I’m not sure if this is something that can be improved through instructions or customized GPTs, or even in projects. But I’m not currently able to perform summarys of the demanded length. I came here for advise

Thank you!