Hello masfur,
.png files are bitmap/raster and will not scale larger in size without losing quality, as utnik stated, these images are linked within the .svg file and are not converted to a vector format which you would need to enlarge them and keep the quality. Are these images more line drawings or a true photograph image? Could you share one or two of them so I can better recommend some options to convert them to vector format if the image seems like it would be feasible to do so. Thanks!
Cheers!
mnawij