This happens because the avatar size requested in that template is “huge”
“huge” here means 120px for normal DPI screens and 240px for high DPI screens.
If you want to request 500px avatars, you need to override the template here
you can do it like so (in the Header section of your theme)
<script type="text/x-handlebars" data-template-name="components/user-profile-avatar">
<div class='user-profile-avatar'>
{{bound-avatar user "500"}}
{{#if user.primary_group_name}}
{{avatar-flair
flairURL=user.primary_group_flair_url
flairBgColor=user.primary_group_flair_bg_color
flairColor=user.primary_group_flair_color
groupName=user.primary_group_name}}
{{/if}}
{{plugin-outlet name="user-profile-avatar-flair" args=(hash model=user) tagName='div'}}
</div>
</script>
You can read more about overriding Discourse templates here
Please note that if you override a template, and it changes in Discourse core, you need to update your override.