Perhaps because Majunior and Daimao are different entities, only with similar qualities and the shared connection to kami. We know that Kami was a Dragon Clan prodigy (and thus PD is too), but Piccolo Jr. was a fighter type Namekian meaning he had a different body type.
Also @Fantastische Hure that's a good explanation, but he never accesses his true power after rejuvenating until fighting Goku, so it might just be an issue with him overall from before he was sealed in the Mafuba jar. Perhaps his FP strains him so much because when Kami split, he kept the bulk of his power for himself while Daimao was obviously far weaker based on old Kami >> Popo >> young Daimao?