Why do I usually take selfies in front of the mirror with a round face and a small face, when it is obvious that someone else took a SLR for me?

This may be caused by the different viewing angles and focal lengths of SLR cameras and reflectors.

First of all, the angle of view of the mirror is usually round, which will make your face look softer and more evenly distributed. The angle of view of a SLR camera is rectangular, which will make your face look more square in the photo.

Secondly, the focal length of a mirror is usually shorter than that of a mobile phone selfie, which will make your face look more compact and symmetrical. SLR cameras usually have a longer focal length than mobile phones, which will make your facial features more prominent, such as facial contours and lines.

Finally, the angle and distance of the photo will also affect your face shape in the photo. If you don't find the best angle and distance when taking pictures with a SLR camera, the five senses in the picture may be more prominent and make your face look more square.

To sum up, different camera equipment, shooting angle and distance will affect your appearance in the photo. So, sometimes what you see in the mirror is different from what you see in the photo.