为什么nii.gz转.nrrd标签体积变大？

import SimpleITK as sitk
# nii nii.gz nrrd格式之间互相转换
def nii2nii(oripath, savepath):data = sitk.ReadImage(oripath)img = sitk.GetArrayFromImage(data)out = sitk.GetImageFromArray(img)sitk.WriteImage(out, savepath)if __name__ == '__main__':oripath = '00292625.nii.gz'savepath = '00292625.nrrd'nii2nii(oripath, savepath)

几百kb变成四五百MB，什么原因导致体积变大的？有知道的大牛请指教！谢谢