Is the interaction resulting in the champ with hunter doing full normal damage to the colossal champ, so the abilities effectively cancel each other out? If so, then I'm ok with it working like that. I haven't faced the pairing enough from either end so far to have a stronger opinion on it yet though, so this is just an initial opinion.
I personally believe Colossal is a bit too strong and needs a weakness. Making Hunter: Giant ignore Colossal would both be good for balance and great flavorwise.
if a large champ with colossal and hunter giant has its own attack redirected onto itself, how much dmg should it do?
Would be best fitted into Colossal itself, like: This champion takes 50% less damage (after DEF) from small champions without Hunter: Giant.
unless of course applying hunter: giant from the same champ to both the attack and the defense violates the rule against stacking