Xiao Ying said that the cow is thin because it gave birth to a calf and is sick. This statement makes sense as giving birth can be a physically demanding process for the cow, and it may lose weight in the aftermath. Additionally, being sick can also cause a loss of appetite and lead to weight loss. However, it is important to ensure that the cow receives proper care and treatment to help it recover and regain its health. This may include providing appropriate nutrition, medical attention, and a comfortable living environment.