According to Serebii:They're the same it's just the angle serebii takes the screenshots at is really stupid, plus lets try to not make some trivial thing as this so important to the discussion
Latios: 6'07", 132.3 lbs
Mega Latios: 7'07", 154.3 lbs
Latias: 4'07", 88.2 lbs
Mega Latias: 5'11", 114.6 lbs
They're not the same size.