A virtual machine snapshot file is a growable file, that is, it employs a copy-on-write (COW) mechanism, hence it starts small and with every write it can grow as much as the base disk. This also brings about an inherent performance penalty. A read has to traverse the entire snapshot chain to reference the correct block and a write has to get a block allocated and then write even if the parent/base disk is eager-zeroed thick format.

For best practices on creating virtual machine snapshots, read the VMware Knowledge Base article 1025279: https://kb.vmware.com/s/article/1025279.