Any time you switch themes in WordPress you need to generate all of the images sizes registered in the new theme.
If you switch to Mai Theme and notice your images (most noticeable on blog archives or in Mai Grid blocks) are not displaying at the same size, this is very likely the cause.
Regenerate Thumbnails plugin is extremely popular and has been very reliable.
- Install and activate Regenerate Thumbnails plugin.
- Visit Dashboard > Tools > Regenerate Thumbnails.
- Choose the options you’d like. The defaults are typically enough, but you may like some of the options available in the plugin.
- Click the Regenerate Images button. Regenerating only featured images may work, but it’s often best to regenerate all images, since Mai Theme has some image performance improvements in place for cover blocks and other areas of the theme/site.