To reduce HTML email file size and prevent clipping in Gmail (which occurs at 102KB), the primary strategies involve optimizing code, images, and content. Code optimization includes minimizing whitespace, avoiding nested tables, using CSS (especially external stylesheets) instead of inline styles, and inlining only critical CSS. Image optimization focuses on compression, using appropriate formats (JPEG for photos, PNG for graphics), and setting proper dimensions. Content reduction can be achieved by using preheader text effectively, reducing the number of fonts, and employing modular design to reuse code blocks. Avoiding excessive background images and using HTML minification tools are also helpful.