I think the key thing with Powerful Build is that it's only granting you some of the benefits of being Large, without the character actually being Large; basically you can carry/drag/lift real good, and that's it. There should be no expectation that you will receive any further benefits as you're still only a Medium sized creature.

If you want more of these benefits then you need to actually become a Large creature somehow, i.e- be a Rune Knight and use Giant's Might, use the enlarge/reduce spell (or have someone use it on you) and so-on. These actually mean that you're the correct size to wield a large sized weapon properly, as it's not simply a matter of carrying capacity (as even a Small creature can lift a seriously large weapon with a high enough Strength score, but that doesn't mean it'll be of any use to them).

Now, about enlarge/reduce, it clearly states it's ruling in the spell own wording and thus changes the rules as written in the DMG about oversized weapon. It clearly states that your weapon deals an extra 1d4. Thus it override the general rule that a larger sized weapon deals an extra die of damage.

Late to the party but I do want to respond to this because if you read carefully you can see this is actually wrong. 
" The target's weapons also grow to match its new size. While these weapons are enlarged, the target's attacks with them deal 1d4 extra damage."

This means that if you have an oversized weapon already on you, it doesn't grow in size to match the target, because it already is the right size for the target. 
Since the weapon is not enlarged, it doesn't deal the extra 1d4 damage, and thus you can count the oversized damage multiplier instead.

No, that doesn't quite work. In this case, the logic for the disadvantage is the proportions of the weapon rather than its weight per-say. Basically it's a bit unruly and difficult to swing around because of that.

But, if you're trying to make this work as a gimmick or a character, what you can do is make a character with at least 2 levels in Rogue and 2 levels in Barbarian. A 2nd level Barbarian gets Reckless Attack which gives you advantage on attack roles usually in exchange for enemies having advantage to hit you next turn. This makes it to where the Disaddvantage from the Oversized Weapon is canceled out. Then the 2 levels of Rogue give you Cunning Action which let's you make a Bonus Action to, in this case, take the Dodge Action, canceling out the Advantage Reckless Attack would've given enemies to hit you. 

Do it like this and you can flavor the character to be able to flawlessly weild a massive weapon by your 4th level.

Quote from KaiYamazaki>>

But, if you're trying to make this work as a gimmick or a character, what you can do is make a character with at least 2 levels in Rogue and 2 levels in Barbarian. A 2nd level Barbarian gets Reckless Attack which gives you advantage on attack roles usually in exchange for enemies having advantage to hit you next turn. This makes it to where the Disaddvantage from the Oversized Weapon is canceled out.

I don't know what post you're responding to, but you're conflating the Disadvantage Small creatures take when wielding a Heavy weapon with a rules we simply don't have for wielding an "Oversized" weapon. 

You are fundamentally correct that Powerful Build is about carrying capacity (it doubles your Strength for determining how much you can carry), which the 5E rules do not correlate with object dimensions (longer objects aren't more awkward to carry), while oversized weapons are challenging to wield presumably primarily due to their dimensions, not their weight, or we'd have generic rules for wielding them via Strength like we do for jumping.  A much more interesting question for your DM, I feel, is, if you're not using variant encumbrance, whether or not Powerful Build should help you wear heavy armor like it does if you do use variant encumbrance.

Then the 2 levels of Rogue give you Cunning Action which let's you make a Bonus Action to, in this case, take the Dodge Action, canceling out the Advantage Reckless Attack would've given enemies to hit you. 

This is illegal.  Rogues can only use Cunning Action to Hide, Dash, or Disengage.  Monks can Bonus Action Dodge, but it consumes Ki.

Bu’ if tha halfling had pouwerful built?

Weirdly, it wouldn't do anything, since the halfling would then count as medium-sized and both small- and medium-sized creatures get 1x the normal amount of carrying capacity (15x STR for carrying, or double that for dragging, lifting, or pushing.

I know it’s not RAW, but as a DM I rule that Small creatures with Powerful Build can wield weapons with the Heavy property without restriction.

Is there even a canon way for a halfling to get Powerful Build?

Quote from 6thLyranGuard >>

Is there even a canon way for a halfling to get Powerful Build?

 
That’s a great question. Is it possible from a subclass? I use a lot of homebrew, so there’s that.

I can't think of any subclasses that grant it.

Rune Knight has the ability to enlarge themselves temporarily, and some spellcasting classes can take enlarge/reduce, but those would be the only official options IIRC, there's no canon way to get Powerful Build specifically outside of a race.

If you want to go homebrew I'd speak with the DM about maybe a ring or something? Powerful Build really is more of a utility feature so I don't think it'd be especially broken to get it, just keep in mind that many small races have quite strong traits to compensate for the disadvantages of being small (though really heavy is the only truly mechanical one IMO).

your a chad! in the nicest way