Some example Awareness Test modifiers (pg 96 of the Black Crusade Core Rules, and pg 118 of the Only War Core Rules) include +20 to notice the passage of a Hulking Creature. However, even though Legion Power Armour raises the wearer's Size by one step (to Hulking), Chaos Space Marines possess the Black Carapace, "…meaning they do not count their Size as one step higher."
Pg 18 of the BC Core Rules and pg 32 of the OW Core Rules states under the heading TEST DIFFICULTY: "While the difficulty of a given test is occasionally clearly stated by the rules, often it is left to the GM to determine just how difficult an action is."
So perhaps I don't award a +20 bonus to an Awareness Test to detect the Chaos Space Marine because he is Hulking, but instead award the bonus because his armour's power supply and servos hum and whine, and because the overlapping fit of some sections make noise (such as the pauldron against the cuirass, or the poleyn against the cuisse). Throb-hum, whir, click, clack!
Unless he's carrying a Stummer…
Awareness opposes and is opposed by Stealth, and is a [quote] "Free Action in reaction to something." Out of all the Adversaries listed in the BC Core Rules, only nine of them don't have Awareness; Administratum Adept, Cackling Abomination, and Ax'senaea the Thrice-Possessed (WTH!?) to name three, and that last one still has a 44% after the unksilled penalty. I think it's safe to say if a CSM doesn't have Stealth, chances are he's likely to be noticed.
As a GM, I'd be inclined to rule movement more than Half results in a penalty to Stealth: Full Move -10, Charge -20, Run -30. I'd also be inclined to have the Player make two Stealth Tests, even though it is a Free Action as part of Movement: once at the beginning of the move (to represent possibly gaffing straight out of the gate), and the second at the halfway point (to represent maintaining Stealth discipline). Do you want to be sneaky, or do you want to get there quickly? Doing this might add some immersion tension to the session.
So, perhaps because it is "often left to the GM to determine just how difficult an action is", having an additional penalty resulting from a high AP value might be considered overkill.
There is also this to consider:
Black Crusade lists many Armour Upgrades, none of which are available in Only War, and the developers may have considered the additional weight these could potentially add to Armour as penalty enough without piling on another -30 to Stealth Tests?