The MacBook air is only 1.3kg. And it really doesn't take up a lot of space. No more than an A4 notebook. If you're keeping it in a bag, then I think it ought to be small enough.
Edit: I exagerated. Should have been more clear. A notebook that you would carry 12 hours or more with you is not a A4 notebook that weighs 1.3kg. It is something like this: https://www.amazon.com/AmazonBasics-NH130210120V-R-Classic-N... - 8" and 350g
https://www.amazon.co.uk/Pukka-Metallic-Jotta-Wirebound-Note...