Alex, why do you measure the strength of an army in hit points? Isn't that a good indicator? :confused:
What are 9 basilisks and 50 gnolls worth? It's nothing! But an angel has both stats and speed. A ranger comes with elves, not gnolls... by the time they crawl over, they'll be shot down. And with Haste, an angel can eliminate the elves in the first turn.

Not exactly strength, but rather "bulk," because magic doesn't care whether an angel has 20 defense or a gnoll has 6. For example, with heroes like Neal or other magic-focused heroes, it's often easier to kill 4 angels than an army of 50 gnolls, 9 basilisks, 5 wyverns, 7+1+1 sprites, and other minor units.
Even a barbarian with Implosion will take out at least 3 angels at a time... ;)
Of course, it's a debatable issue... and depends on the situation, but still, the trade is unequal in favor of hit points, but it's effective in terms of a powerful stack – it's just a trade, not an increase.
If we consider Consumption as a 100% bonus, we'll get, say, 25% of the effect in this case, while the opponent, for example, uses Rampart or Castle to gain Consumption 3 or 4 and gets about 70-80% of the effect.
P.S. Trading troops will always be worse than gaining an advantage with minimal losses.
And as Qwerty rightly pointed out:
Armies have the characteristics and artifacts of the hero... and a 3-13 parameter increase has a greater effect on a gnoll than on an angel (doesn't it?).
A gnoll becomes 8-19, and an angel 23-33... the difference is no longer 4 times, but 2 times!
And if there are also artifacts for hit points!
And we shouldn't forget about the effect of secondary skills, as they further multiply the effect on numerous troops...
How much damage does 1 angel deal on average? 50-65
and 50 gnolls?
80-130.