Created attachment 191192 [details]
A slide in A6 portrait format showing a circle 5cm x 5cm exported to PNG

In same cases, the size ratio of a slide exported to png file format is modified. It happens when the format of the slide has margins not null.
The attached png file is created by exporting a slide in format A6 portrait with margins set to 1cm (default margins). On the slide, the figure is a circle of size 5cm x 5cm.

No distortion appears when the margins are set to null or when the slide (with or without margins) is exported to jpg file format.

Please note that the behavior reported is not due to changes of margins value.
(which could be another problem). The figure on the slide is a circle just before and after exporting it (checked on position and size tab).

My Steps to get the png file attached
File > New > Presentation
Slide > Layout > Blank Slide
RMB (right mouse button) on the slide > Slide properties
Select ‘Slide’ tab if it is not already selected
Choose Paper Format A6 portrait 
Check if the 4 margins are set to 1cm. If not set them to 1cm.
LMB (left mouse button) on OK button

LMB on Ellipse button
LMB on the slide (near the center)
RMB on the small circle > Position and Size
Set the size base point to the center
Set “keep ratio”
Set “Width” to 5cm
LMB on OK button

File > Export
Select PNG on drop down menu ‘Save as type’ and save

For comparison purpose

File > Export
Select JPEG on drop down menu ‘Save as type’ and save