Apple MacBook Air battery

I noticed a very interesting aspect about the MacBook air's battery over the years. I bought a MacBook Air 2013, and recently I replaced it with the new MacBook Air 2018 13 inch Retina display.

I ran coconut battery on both devices, and surprisingly the 2013 MacBook Air had a design capacity of 7150 mah, while my 2018 MacBook Air has a design capacity of only 4381 mah!! Both my macbook's battery are in perfect condition.

So what is going on here? Obviously every year the MacBook gets better battery life, so is the battery usage just more efficient as time goes on? Thanks for any reply.

EDIT: I just have a bad feeling my new MacBook has a defect. Does anyone else have the 2018 MacBook Air and have a bigger battery? If so I'm returning this computer for sure.

I also just upgraded my old MacBook Air 2013 (cpu i7-4650U) with MacBook Air 2019 13' (cpu i5-8210Y) and mine new mac shows me 4381 mAh too. I think Apple reduced the battery capacity to decrease physical battery size. Why? If we'll compare this 2 CPUs: i7-4650U and i5-8210Y we'll see that max TDP (aprox. shows cpu power consumption) is 7W while for i7-4650U it's 15W which is more than twice more. This can explain why Apple reduced the battery capacity almost twice.

CPUs comparison: https://www.cpubenchmark.net/compare/Intel-i5-8210Y-vs-Intel-i7-4650U/3402vs1955