Here’s the thing. If the action should succeed (giant picking up a horse) then why are we even checking the rules? They’re for handling uncertainty. If there is none, then why are we checking the rules?
This is where we can get silly but we shouldn’t. Uncertainty is when things of roughly equal power go up against each other and the outcome is unknown. So hill giant strength is compared to the strength of similar creatures for resolving uncertainty, not for creatures in wildly different power brackets.
Or would DMs here allow an arm wrestling contest between a halfling and a hill giant to be resolved by dice?
Let’s get a grip on things! (so to speak...)
Using ability scores to adjudicate opposed roles is not always simple when there is a significant size Yes, it might be difficult for the giant to grab that pesky halfling but once the giant has a good grasp getting out of that grip should be about as easy as breaking free from hardened concrete. There are exceptions to every rule of course.
I'm not overly concerned about the rules when it comes to carrying things, if I think the kraken can lift the ship out of the water and toss it aside like a toy it happens. Sometimes the rules need to be set aside for the fiction.