Dear respected Enrico Romagnoli,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
I’m sorry you’re having a problem with PowerPoint size but based on your description we can work together to resolve it.
For the situation you encountered, we deeply understand the inconvenience caused and apologize for it.
As per your description, it seems that your PowerPoint file size does not reduce even you removed all pictures and use PowerPoint compress pictures feature.
Sometimes Embedded fonts can be cause large. Sometime go to Master slide layout and re-arrand the layout will reduce the size.
May I know does issue happened do specific PowerPoint document file? If the issue only happens to a specific PowerPoint file, in this scenario, please copy/paste content to a new created PowerPoint file, then to check the outcomes.
Meanwhile, if you don’t mind and if it is convenient for you, could you please send copy of your document file to me so that I can look from my side, and I will check this behavior and help you to fix and verify the result with you.
I have initiate private message for you, if there is confidential information, then you can send it to me in Private Message (PM). you can access PM from the top right corner of this page, as shown in image below:

We appreciate your understanding that sometimes the initial suggestions may not resolve the problem very soon. However, we can work together to narrow down and resolve the situation. So, please kindly provide more information below to let us work further.
Appreciate your patience and understanding and thank you for your time and cooperation.
Sincerely,
Waqas Muhammad