I don't understand the use case for setting a remark on a key signature with the empty string. It seems more likely that any empty strings passed into remark are just being populated from (for example) the default value of a graphical input field that the user left idle.

Applying the following patch (against master) should reduce the size of certifications made via the Qt bindings (untested, sorry!):

--- a/lang/qt/src/qgpgmesignkeyjob.cpp
+++ b/lang/qt/src/qgpgmesignkeyjob.cpp
@@ -83,7 +83,7 @@ static QGpgMESignKeyJob::result_type sign_key(Context *ctx, const Key &key, cons
         skei->setDupeOk(true);
     }
 
-    if (!remark.isNull()) {
+    if (!remark.isNull() && !remark.isEmpty()) {
         ctx->addSignatureNotation("rem@gnupg.org", remark.toUtf8().constData());
     }