With an absolute primogeniture you will see significantly less situations where dynasties enlarge their holdings by marriage. In fact it probably will only happen when at least participant is a sole child. Marriages between firstborns will be either strongly discouraged, or alternatively result in a "junior" partner in the marriage (one who abandons their family name) disinheriting themselves.
Also if RL history is any indication nobody ever though it is possible to have too much land. So splitting will not be done because the lands are "unwieldy", you just hire stewards. It can be done because parents wanted to give a good life (and possibly political power) to all of their children, but depending on time and place it could be against the law (at least as far as the land and buildings go, giving out gold, animals etc was probably legal everywhere).