We often face this issue wherein the size of a powerpoint is more than what we expect. This often leads to delay in sending your file over outlook or any other e-mail platform.
Cause:
While creating a new powerpoint we use templates in our library. These templates may be customized or heavily designed which we can not make out until we realize the size of the powerpoint is too large. These heavily designed slides are not visible in your powerpoint and you may or may not require them, but these slides are the main cause of increasing the size of PPT.

This powerpoint has just a single slide yet the size is around 3 MB, WHY?
Often we copy slides from another powerpoint and create a new one out of that version taking the other one as a template. This increases the size since the template was heavily designed or had slides with heavy content.
Resolution:
To reduce the powerpoint size:
Step 1: Move to the Review Section –> Slide Master
Step 2: Remove slides which are not in use:
RESULT:

The size of the ppt reduced from 3.1 MB to 28 KB after removing the unused slides in the Slide Master.