The character carrying a load of equipment is doing so because they are strong enough to do so without the weight of it impeding their ability - that's the metric that a strength score describes, and the carry-weight realism relation is marked out by that relationship - so if you're not carrying your encumbrance limit, then the weight of what you're carrying isn't enough to have a notable impact on your abilities. Carry enough that you dip into the yellow, and suddenly Astarion with no encumbrance *CAN* jump further than you... further more, if Astarion tried to carry as much as you are he couldn't jump at all, and could barely even walk under... because you are stronger than him. This makes tractable sense, and if you were keeping track of it in a table top situation and using the handbook values for variant encumbrance to give it more realism, then you'd find out that, actually, it makes rather a lot of real-world coherency (at least while dealing with Medium creatures).
Dex Based acrobatics doesn't let you jump further or higher - it lets you jump more precisely and retain your balance to jump again in quick succession and perform other related feats of dexterity and acrobatic skill. Real-world acrobats do not make shows and displays of how far or how high they can jump - they make shows of how artistically they can control their body through complicated movements and precise action and balance. The real-world people who make shows of how far and how high they can jump are olympians... and the acts of jumping far and high fall into a specific olympic category - Athletics, incidentally, not Acrobatics, which is a different olympic discipline...
It IS nice to have a lithe and dexterous fighter leap about from balcony to chandelier to opposite balcony with dramatic flair! That's a great use of acrobatics, because the individual distances are not challenging, and the trick is in the balance and precision of the activity. The Barbarian running up behind doesn't do this - The barbarian runs at the edge, ploughs through the balcony in one great leap, clotheslines the chandelier as he goes past and crashes through the opposite balcony to land on the other side all in one go - it's not graceful or dexterous, it's purely athletic. This is a perfect example of where both athletics and acrobatics might allow a character to make the crossing, from one balcony to the other - and most DMs would have no problem with the dex player asking for acrobatics and describing their balcony-hoping, chandelier-swinging crossing. If there WAS no chandelier, however, then I'm sorry, but it doesn't MATTER how graceful and dexterous and well-balanced you are, or how perfectly precisely control of your own body you are - that's a straight jump and you're going to have to put some raw physical oomph behind trying to make it the distance: that's Athletics.
