Apple Launched its new MacBook on 10 November, 2020. It uses in house made M1 processor. Previous generation of MacBook has Intel processor. Apple has to transition because Intel was not providing power efficient processors according to Apple's requirement.
This transition uses new M1 chip which has processor, graphics as well as memory. This results into less components on motherboard.
After observing difference between motherboard of MacBook with Intel processor and M1 chip. There is significant reduction in components on motherboard of latest MacBook. Due to reduction in amount of components these has lead to empty spaces. These empty space could also be below coolant. These empty spaces could result in reduction in size of motherboard and in turn reducing the dimensions of device.
For comparison take a look at motherboard which has intel processor. It is completely pack with little to no room for improvement.
These could also result in increasing battery. Baseline MacBook has around 5k mAh and high end MacBook has 8k mAh. In next generations of MacBook this might increase. In airplanes there is limit of 27k mAh at 3.7V, which is 100Wh. So there is definitely some margin for increasing battery of baseline as well as high end MacBook.
Despite Apple has been some years behind in showing improvements in MacBook in comparison to its other devices the next generations of MacBook will have lot to offer.