I have a Canon 6D, an iPad Air and Lightroom 5.5 in my Mac.
While travelling I shot some JPG photos.
Imported them with the SD Card reader into my iPad and then imported it to Lightroom Mobile.
Lightroom Mobile Synced with the CC and then my Mac Lightroom downloaded the photos.
Why they have different sizes?
I am not sure what you are asking here. The image you attached shows two different pictures-they would usually be different sizes as they likely do not have identical content.
That explains it! Thank you for you time, I'm sorry for such a lame mistake!