Subject and body are URL-encoded
An email QR is built by appending the subject and body to the mail address. Characters that can't appear in an address as-is — spaces, line breaks, non-English text — get converted through URL encoding. A single space becomes %20.
Because of that conversion, far more characters go into the code than you see on screen. A single Korean character expands to nine characters like %ED%95%9C, so a Korean body grows the code much faster than an English one.
A longer body makes the QR dense fast
More characters mean more modules packed into the same area, and smaller modules are harder for a camera to read. Combine that with the encoding expansion above and a few lines of body text can visibly thicken the code.
Fill in the subject and leave the body empty, or keep the body to a single line like "Tell us what you need." If you have a lot to say, put it on the printed material or a web page and use a URL QR rather than stuffing it into the body — that keeps the code light.
Scanning never sends the email
A scan just opens the device's default mail app with a draft ready. The sender reviews it and presses send, and they can edit the subject and body however they like before doing so.
So don't assume the wording you prefilled is the wording that arrives. On a device with no mail app configured, nothing may happen at all — print the address next to the code as well.