>>17190383While I can't say for certain without you naming a Pokemon lets use something like Snorlax for the sake of discussion.
He has great HP, poor defense, but great special defense. Does he have good bulk? On his physical defense side, no. On his special side, yes.
Can this be remedied? Yes, to an extent. You can go impish nature (+def,-spa) to increase his physical defense, and even give him +252 def to further increase his bulkyness in his defense, or you could run a curse set that raises his defense 1 stage each time you use it.
The big picture is, bi and large, there is diminishing returns when you're dealing with defense values in Pokemon. All attacks do at least 1 damage, even if it's impossible to calculate, while at most the attack could do sufficient damage to OHKO a Pokemon.
Basically, what I'm saying is, if you want a mixed wall for something like Rotom Wash, generally it's best to have a good mix of both defenses and HP. The generic set for Rotom Wash has 252 HP and then 252 SPA, DEF, or SPD because HP is what Rotom wash lacks the most. It has great bulk on both its physical side and special side, but it can't make much use of it with a small HP pool.
That's a really long explanation for a rather basic question, but yeah...
tl;dr - unless you want a pure wall for physical or special defense, you want both good hp and defenses, keeping in mind diminishing returns is a thing in pkm
