WordPress provides three default image sizes (large, medium, thumbnail) based on global media settings, but these often don’t align with your block theme’s actual layout dimensions. This creates performance problems when users select inappropriate sizes.
The issue becomes apparent when content width is 620 pixels but users select “large” images at 1024 pixels. While browsers resize images visually, you’re still downloading unnecessary data and wasting bandwidth.
Default WordPress image sizes were designed for generic use cases, not the specific content and wide sizes that modern block themes use. This mismatch between available options and actual layout needs creates suboptimal user experiences.
Understanding this fundamental problem helps explain why custom image size configuration is essential for performant block themes rather than relying on WordPress defaults.