It happens with me when attaching images. Their total size is much less than 25MB. Today it happens with images with total size of 7MB.
There is a bug report in google support center from 2020. The bug is still not fixed.
It seems as a workaround to "drag and drop" files into the area of your email. This would include the files inline.
Not the real solution but it works. Maybe we will need to wait few more years Google to fix this in GMail.