User resources retrieved over REST (e.g. via https://jira.atlassian.com/rest/api/latest/user?username=rfernandes ) return avatar URLs like:

avatarUrls: {
16x16: "https://jira.atlassian.com/secure/useravatar?size=xsmall&ownerId=rfernandes&avatarId=19490"
24x24: "https://jira.atlassian.com/secure/useravatar?size=small&ownerId=rfernandes&avatarId=19490"
32x32: "https://jira.atlassian.com/secure/useravatar?size=medium&ownerId=rfernandes&avatarId=19490"
48x48: "https://jira.atlassian.com/secure/useravatar?ownerId=rfernandes&avatarId=19490"
}

However, the 24x24 ("size=small") and 16x16 ("size=xsmall") URLs both link to the same 16 by 16 pixel image.

This seems to apply to user uploaded avatars only. For example, the following built-in avatars are correctly sized:

• 16x16 : https://jira.atlassian.com/secure/useravatar?size=xsmall&avatarId=10613
• 24x24 : https://jira.atlassian.com/secure/useravatar?size=small&avatarId=10613

... but the following uploaded avatars aren't:

• 16x16 : https://jira.atlassian.com/secure/useravatar?size=xsmall&ownerId=rfernandes&avatarId=19490
• 24x24 (actually 16x16): https://jira.atlassian.com/secure/useravatar?size=small&ownerId=rfernandes&avatarId=19490

(Side note: it seems that most uses of a 24x24 avatar in JIRA's UI use the 16x16 image, and scale it to 24x24 via CSS or image attributes, resulting in slightly degraded quality.)