Unlike most pro heroes in My Hero Academia, Momo Yaoyorozu's Quirk, Creation allows her to travel light without ever worrying about supplies. Able to make make everything and the kitchen sink, her hero name, The Everything Hero: Creati makes a lot of sense. Much like when she creates something with her Quirk, Yaoyorozu has been keeping a few secrets and waiting for just the right time to expose them -- and that time is now.
She's Much Heavier Than She Looks
Creation primarily works by transforming Yaoyorozu's lipids (essentially energy stored as fat, oil and other substances in the body) into various other atoms, allowing her to reconstruct items at the molecular level. What this means is that Yaoyorozu is incapable of becoming overweight. Any fat she stores can be used in bulk, allowing her to literally shed pounds in seconds.
As such, Yaoyorozu probably watches her weight constantly to make sure that she's not stripping her body entirely of fat, which could have serious detrimental health effects. Couple this with the law of conservation of mass -- and the fact that she's able to pull cannons out of her body as shown during the Heroes Rising movie -- and it's clear that despite her slim figure, Yaoyorozu's pretty hefty. Even if we conservatively estimate the cannons at 170 pounds each, it's clear that a punch from a pre-Creation-using Yaoyorozu should hit like a freight train.
Creation Forces Her to Eat A Ton
Due to her body's reliance on having a vast store of lipids to draw from in order to transfigure those building blocks into their new atomic structure, Yaoyorozu has no choice but to be a big eater. If she doesn't eat enough, she may find herself too short on materials to create whatever she needs for battle. As a result, she can be both completely free and super-strict about her diet. The saying "too much of a good thing" still applies to her after all, and if she eats too much of an item, she may end up giving herself a vitamin toxicity, causing unwanted side effects. While this is unlikely to occur given the high metabolism that enables her Quirk to work, it still necessitates a balanced diet -- until dessert. Since she requires lots of fat to use her Quirk, proven fattening foods are her bread and butter, allowing her to stock up on the lipids she needs without overeating.
She Needs a Lot of Surface Area to Draw From
Yaoyorozu's ability to create almost anything is incredibly useful in My Hero Academia -- but can be difficult to use. As she demonstrated during the attack on the Unforeseen Simulation Joint, in order for large items (such as a large insulating blanket) to exit her body, she has to make use of the full surface area of her skin. Whatever she creates will come bursting out regardless of the clothing she happens to be wearing.
Small or thin objects like swords or roller skates can be drawn from the arms or shoulder area, but she tends to prefer her back or side for larger items. This is a huge hindrance in low-temperature environments, where not only does her Quirk require her to expose her skin to the open air, but by drawing from her fat stores, she also removes any insulation "blubber" it may have given her. In contrast, working in high-temperature areas wouldn't necessarily be ideal either as the dry air and sun could easily crack and burn any exposed skin if she neglects to make sunscreen.
She Builds Items At The Molecular Level and Stores Them For Later
When it comes to Creation, the Quirk's official description as given in Volume 2, Chapter 16 is "She can create anything as long as it is nonliving!! But she needs to understand the object's chemical makeup first." As any chemistry student can attest, things can get complicated quickly. Even simple hydrocarbons are so unwieldy to "draw" that a shorthand known as the Carbon Skeleton Formula -- which reduces the compound to a series of lines and symbols -- was created.
Keep in mind that these are just the basic formulas like propane and butane, nothing like the more complicated Matryoshka dolls Yaoyorozu creates in Volume 8 with various paint colors and flash-grenade payloads. To be able to understand the purpose of every aspect of an item on its atomic level -- then store that knowledge and call upon it in battle without messing up -- is a superpower all its own, putting other intellect-based superheroes to shame.
The Larger the Object, the More Time, Concentration and Multitasking It Takes
Constructing a fully operating flash-grenade from scratch -- let alone at its molecular components -- is hard enough, but Yaoyorozu is usually working on much bigger items like giant shields while on the battlefield. In addition to this monumental task, she usually finds herself taking charge, organizing and executing plans that use others' strengths and abilities. Thus it's amazing that before she even begins telling the others about a plan, she has to begin prepping any large set-pieces she needs to ensure its success -- even if Yaoyorozu herself is defeated.
While she does reveal to her My Hero Academia allies that it takes a while to prep these items, often asking them to buy her time, the girls' ability to multitask helps Yaoyorozu deceive her enemies, ensuring they believe she can pull out anything she wants whenever she wants. This tidbit of psychological warfare keeps enemies cautious when approaching her. If they were to rush her and go the brute-strength route, there's a possibility that she could be knocked out before she has a chance to implement her plan -- or be of help to anyone.
