Why it increases size when i already saved the design JPG?

[Mahmoud Elgohary97]

[c.pfaffenbichler]

My recommendation: Do not re-save jpgs because of the cumulative effect of the lossy compression. 

As for your observation: 

• The Image Size dialog gives a number for the uncompressed flat image (so the number depends on Color Mode, Bit Depth, number of pixels and disregards Layers etc.). 

• The number the jpg dialog gives is an estimate, so meaningless. 

[Mahmoud Elgohary97]

So what's your recommendation for saving the design to post it on social
media?

[c.pfaffenbichler]

Saving a jpg-copy off of the layered image is fine, just try not to open/edit/save jpgs. The image-quality would be decresed (slightly or not so slightly) for each time one does that. 

Your original description was not completely clear, so maybe that’s not what you are doing anyway.