Vector formats are images in geometric separate shapes such as curves or polygons, as the size increases the program recalculates each piece keeping the proportion and the resolution intact for a crisp image.
A raster graphics image, or bitmap, is a data file represented generally by rectangular grid of pixels. If you increase the size of the image the program just increase the size of the pixels in the image, making it blurred or distorted. Raster graphics cannot be scaled to a higher resolution without losing quality. Raster files are .bpm, .jpg, .tiff, ,psd, and ,pict.
DPI or PPI
Pixels per inch, also called “dpi” (dots per inch). It is the amount of pixels per square inch