Big language = lots of speakers, and small language = few speakers. In Finnish, that is a somewhat common way to express a relative 'size' of a language.
According to Helsinki university, there are about 7000 languages, the median number of speakers of a language is only 7500, and the 23 'biggest' languages cover about half of the world's population.
They're pretty much interchangeable, similar to English. 'Suuri' could be preferred in written language, and 'iso' is maybe more common in spoken language, though the former is used too.
'Suuri' could also be bigger/larger than 'iso', like big vs. grand; e.g. we also say 'suurensuuri', meaning something really grand/massive, but there's no such thing as "isoniso".
@Hacu: Isn't "suuri" also more abstract than "iso"? In the sense that with one thing being iso and the other thing pieni one could objectively measure the relative size or weights? While "suuri" can apply to languages and other objects that cannot be measured always? Kysymys: Beethoven was a great composer, and he sure was "pieni," but as a composer he was "suuri" and not "iso"? (Sept 2020)