Obscuring Mist doesn't care about how much of any given creature is within the mist. It cares about how far you are from the creature you are trying to hit.
If the only way you can hit a Large (or larger) enemy is to be attacking squares that are within the mist, you will have to deal with concealment. If, instead, you are attacking squares of the enemy that are not within the mist, you do not have concealment to deal with.
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This is clearly spelled out in the combat rules in the CRB:
Concealment
To determine whether your target has concealment from your ranged attack, choose a corner of your square. If any line from this corner to any corner of the target's square passes through a square or border that provides concealment, the target has concealment.
When making a melee attack against an adjacent target, your target has concealment if his space is entirely within an effect that grants concealment. When making a melee attack against a target that isn't adjacent to you, use the rules for determining concealment from ranged attacks.
So supposing we have a giant standing halfway in an obscuring mist. For a ranged attack, he'd get concealment against everyone, since you can't draw a line to all corners of his space without passing through the mist.
Against melee attacks, you could attack him without concealment, since not his entire space is inside the effect that grants concealment. (Obviously, this assumes you can actually see the un-concealed portion; it's no good if that's on the wrong side of the mist cloud from you.)