Signing a document with OpenPGP the <PGPKeyPacket> element in META-INF/documentsignatures.xml contains the full public key, which with many signatures easily can blow up to 1.5MB or more. Optionally only the minimal public key should be included, which is stripped of all signatures except the latest self-signatures. Gpgme has a GPGME_EXPORT_MODE_MINIMAL flag for this, see https://www.gnupg.org/documentation/manuals/gpgme/Exporting-Keys.html
Thorsten Behrens committed a patch related to this issue. It has been pushed to "master": http://cgit.freedesktop.org/libreoffice/core/commit/?id=7b00829e27b0a26e9fa8d06bb651134f03a466e9 gpg4libre - tdf#113188 add option for minimal PGPKeyPacket It will be available in 6.0.0. The patch should be included in the daily builds available at http://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More information about daily builds can be found at: http://wiki.documentfoundation.org/Testing_Daily_Builds Affected users are encouraged to test the fix and report feedback.
new hidden config item: Office/Common/Security/OpenPGP/MinimalKeyExport - set to true to get only minimal public key in the key packet
Verified it works as intended.