How are volumes determined in manga?
There is no fixed rule to decide the boundary of a volume of a Manga. It's upto the mangaka and the publisher to decide where a given volume starts or ends. They're generally published arc-wise to maintain a uniformity throughout the volume.
As per Wikipedia

In Japan, manga are usually serialized in large manga magazines, often containing many stories, each presented in a single episode to be continued in the next issue. If the series is successful, collected chapters may be republished in tankōbon volumes, frequently but not exclusively, paperback books.
After a series has run for a while, publishers often collect the episodes together and print them in dedicated book-sized volumes, called tankōbon.