The imaging data in STCs (slice time courses of FMRs) and VTCs is stored as 16-bit integer data, MTCs (surface related mesh time courses) internally make use of 32-bit single precision floats.
FMR/STCs
For each slice, one STC file is created, covering one acquired slice over time. Hence a functional run with a 64x64 matrix, 30 slices and 250 volumes leads to
2 x 64 x 64 x 30 x 250 bytes = 58.6 MB
VTCs
The same logic applies here, but due to isometric resampling (the default resolution is 3mm in each spatial dimension) and brain-only volumes, the calculation is slightly different:
2 x 58 x 40 x 46 * 250 bytes = 50.9 MB
If you resample the VTC to 2 x 2 x 2 mm resolution you'll end up with 171.7 MB, and for a 1x1x1mm resolution you even would get to 1.374 GB!
It it hence not advised to resample your functional data any lower than your physical resolution!
MTCs
Usually, you would take the step to resample the sphere, getting to the standard number of 40,962 vertices per hemisphere. Since the VTC time courses are then resampled in each vertex, both hemispheres would require
4 x 2 x 40962 x 250 bytes = 78.1 MB