Sizing of Featured Image on Homepage

Hi I am wondering if you could help me... on my homepage my featured image normally gets resized so you can view the whole image which is perfect but I have just added a new post and it has blown up the image so you only see it partially. All of the food images I have used as featured images are the same size 558 × 744 pixels. Is there anyway I can fix this? Thank you in advance. Lexi dontmilkit.uk