The percentage convention came from Bearbrick
The percentages come from Medicom Toy's Bearbrick line. 100% is roughly 7cm tall, 400% is roughly 28cm, and 1000% is roughly 70cm.
Other brands borrowed the shorthand loosely. Outside Bearbrick the numbers signal a rough size class rather than an exact height, and plush lines use them even more loosely.
MEGA is a brand name, not a size class
MEGA is a POP MART line name, not an industry term for large figures. Asking a factory for a MEGA figure will not produce a useful quote, and it borrows a name that is not yours to use.
Describe the product instead. Large-format collectible figure, seated, 28cm, with a display box.
Specify in centimetres and grams
A quotable brief gives height in centimetres, an approximate weight, the material, the finish and the packaging format. Percentages alone leave every one of those open.
Where a buyer wants to match an existing collector size class, state both. Around 28cm, comparable to a 400% figure, is unambiguous.
Size drives everything downstream
Size changes tooling cost, moulding time, paint operations, box structure, carton count and freight. A jump from 100% to 400% is not four times the price. It is a different product with a different production plan.
Decide the size tier before you compare suppliers. Requoting a size change late usually costs more than the sample would have.