Thanks for the feedback!
I discussed this with TStar and others in chat -- they suggested a stat sum of 6 for unupped and 9 for upped, like the other spiders (3|3, 4|2; 6|3, 7|2). As a
creature though, we thought its HP would have to be higher than its ATK, to fit thematically with its element. But a 2|4, 2|7 or a 3|3, 3|6 creature didn't feel quite right to me. From what I've seen people are more likely to value damage potential (ATK) over stalling potential (HP), IMO the reason for all the rushes and firestalls in Arena which focus mostly on dealing damage. So I felt like, cost being equal, the value of a creature with high HP was less than that of a creature with high ATK. That's why I raised the Jupiter Spider's HP so that it exceeded the stat sum that the other two spiders had.
That was just my reasoning though. I'm open to suggestions on how to change the stats.