Exported PDF from InDesign it is dimensionally larger than the InDesign file. Why?

This is what I see

In Acrobat if I go to preferences and go to Page Display

Now the dimensions are not correct in the ruler viewing 

But if you look in the bottom left corner if you hover (there's a preference to have this on all the time)

It shows the correct dimensions

Set to custom resolution

And it's back to correct viewing 

It also appears to be how your System Scaling is set - so I changed my Custom Resolution to 110 to counter balance the system scaling

If your system uses a scaling factor (e.g., 125% or 150% on high-resolution displays), Acrobat may miscalculate the pixel density, leading to incorrect ruler measurements.

Why Not 120 PPI?
Even though 96 × 1.25 = 120, some factors might alter it:

Physical Monitor DPI: Some monitors have a native PPI that's not exactly 96.
Acrobat’s Internal Handling of Scaling: It may round values or apply different scaling logic.
OS Behavior: Windows sometimes applies fractional scaling adjustments that slightly shift the result.

It’s a mix of OS scaling quirks, Acrobat’s handling of DPI, and your specific display’s PPI.

Correcting to specific resolution in Acrobat will resolve your issue. 

Even though the Ruler is a mixed bag of settings - it shows the actual correct dimensions of the document in the bottom left corner. 

Acrobat ruler is showing you the size it is at the resolution set in the Preferences - so to speak.

It doesn't just show a "fixed" measurement like a physical ruler would.
Instead, the ruler shows the calculation of the document based on the display resolution (PPI) set in Preferences and adjusts accordingly.