Looking at the picture from [2]. it looks like the motherboard is really not much much bigger than a smartphone. There's a lot of space dedicated to ventilation (note the two large fans and the black cooling ducts under the hand in the picture. Some fraction of that motherboard is also the solid state storage. The picture here of the internals of an iPhone 11 https://www.zdnet.com/article/heres-whats-inside-your-new-ip... (I don't do videos) sure looks like it's mostly battery. If I discount the empty space, the insides are more battery than not battery.
For instance, in a iPhone 11, the battery occupies less than half of the surface of the phone[1]. On a Macbook pro, it's about the same.[2] [1] see this ifixit video if you want to see it by yourself: https://www.youtube.com/watch?v=Feyqwf3cYtw&t=161s
[2] https://guide-images.cdn.ifixit.com/igi/noPvQ2CLs1jptX2p.hug... You can see that the motherboard is much, much bigger than a smartphone.