It seems natural for products to come in multiple sizes, but dakimakura have the odd distinction of their two primary size options being only 10cm apart from each other.
It’s a weird choice, but there’s a reason for it.
Prior to the 2000s, Japanese dakimakura only came in one size: 160cm long by 50cm wide. This is still the case with many officially released dakimakura covers from companies like COSPA or Kadokawa (Though not always).
During the mid-to-late 2000s, however, popularity of dakimakura began to spread outside Japan, and international demand rose. As e-commerce became better established, accessibility of the dakimakura market increased, and with it, demand rose further.
This started to pose a problem, however.
The shipping weight of a 160cm x 50cm pillow is just above the “standard airmail” threshold for most postal carriers. This made the pillows particularly expensive to ship to other countries, despite being relatively inexpensive products. To solve the problem, manufacturers took 10cm off of the length and started making 150cm x 50cm pillows.
Shaving off 10cm put the pillow just under the “standard airmail” threshold, making them much cheaper to ship internationally and making the market more accessible overall to fans outside of Japan. In addition, the resulting increase in popularity looped back to around to where even some official dakimakura covers are released in the 150cm x 50cm size.
150cm-length dakimakura also became the standard for dakimakura covers produced in the US by independent artists and companies like FurryDakimakura.