In the optional dialogue with Emet, it's explained that Lahabrea was much weaker because he would constantly switch bodies.

>> Choice 1: Why choose this form here in the first?
Emet-Selch: Well, well, what a curious question.
Emet-Selch: Mortal flesh is but the vessel into which we Ascians pour the elixir of our souls, molding it as fits the occasion. Or not, if we so choose.
Emet-Selch: Be it for a year or a millennium, I prefer to retain the same form until my duty is done. So, after arriving here in the First, I fashioned some hapless body into the man you see before you.
Emet-Selch: Though as your friend over there can attest, there are those of us who forgo such alterations.
Emet-Selch: He was ever the rash one, Lahabrea. Jumping from vessel to vessel. Never heeding the toll it took on him.