If it's the established Microverse (and Marvel has several, the Micronauts, the one Jarella is from, this one) then yes, it's another universe. Though the fact that half the worlds look like long chain molecules, and population is tiny relative to humans in the Macroverse (So "big" Bug in the Guardians is a bit of a mystery) etc helped with the "it's smaller, but huge to them" notion.
Shrinking is the standard way of getting there, but there are others, the Prometheus Pit was one and Genis-Vell and Rick Jones spent some time swapping places there via Nega-band.