"Sizeably" is an adverb that means to a significant degree or in a substantial manner. There are several synonyms for this term that could be used interchangeably depending on the context of the sentence. Phrases like considerably, substantially, markedly, significantly, largely, appreciably, and extensively are some of the most commonly used synonyms for sizeably. When describing a big change in growth or an increase in numbers, "proportionately" might also be used. In essence, all these words denote that something is of a noteworthy amount or significance, and they can be used to add clarity and precision to your writing.