A potion of growth or an enlargement spell
By changing to medium size, temporarily, that disadvantage will go away. Both the potion and the spell make that possible.
this point got some discussion early in this edition's release. (This question is related but not identical, as is this one). This little piece of verisimilitude -- which parallels a similar rule about weapons for Large and Huge creatures not getting a damage increase if a medium sized creature uses it (DMG p. 278)-- avoids the absurdity of a halfling wielding a greatsword, while still allowing for a variety of other absurdities.
FWIW, this small sized character can ride a medium sized creature. Gnome and halfling Rangers (for example) can take good advantage of that if they are Beast Masters.
In the interest of Rules as Fun (RAF1)
In making a ruling that is not strictly RAW, consider what is behind the character build, and the use it is making use of Small size. As the DM, consider the impact of ruling that the character counts as Medium for all rules purposes, such that the bulky paladin armor makes up the difference. In that case, a maul-wielding halfling might provide a bit of light comedy, but it would have very little impact on game balance. It's only if the player expected to get all of the benefits of Small and yet avoid any negative consequences of that size that the character concept begins to border on "cheesy" as well as funny. (Thanks to @NeilSlater for this point)
Regardless of what’s on the page or what the designers intended,
D&D is meant to be fun, and the DM is the ringmaster at each game
table. The best DMs shape the game on the fly to bring the most
delight to his or her players. Such DMs aim for RAF, “rules as fun.”
We expect DMs to depart from the rules when running a particular
campaign or when seeking the greatest happiness for a certain group of
players. Sometimes my rules answers will include advice on achieving
the RAF interpretation of a rule for your group. I recommend a
healthy mix of RAW, RAI, and RAF! (Jeremy Crawford, Sage Advice Compendium, page 1-2)