A small group is essentially a two-party encrypted group. If there are 20 people in the group, when Alice sends a group message, she is actually sending 19 separate one-to-one messages (encrypted with the Signal protocol) to the other members. If the relay requires stamps, group messages will consume more stamps. This limits the maximum number of members a small group can support, but a few dozen members are still manageable. In the Signal protocol used for small groups, the ratchet responsible for post-compromise security advances faster than in MLS, so its post-compromise security is stronger.