By the DMG, 138 HP with AC of 15 is squarely in CR 4. Assuming "level" is "CR", if the AC is particularly low, that could be defensive CR 4 in practice. That is supposed to be balanced with the offensive CR, which for CR 4 is 27–32 damage per round -- which as I recall was about right for the phase spider matriarch.

So, eh — it doesn't seem like particular bloat to me.