Thank you for your feedback, @cryptopreneur. We have done this, because we want to put extra emphasis on what is most trending, hot or new. It's actually ranked:
1. The biggest thumbnail is ranked as #1 post
2. The second biggest thumbnails are ranked as #2 and #3
3. Then you have #4, #5, #6, #7
4. The smallest are ranked as #8 and lower.
All new posts have some chance to be a little more in the spotlight. For trending posts that are most popular, they also deserve to be a little more in the spotlight. That's the rational behind the different sizes in thumbnails.
Oh, I see what you mean. Right now the shape is determined by the picture shape that is uploaded. We can definitely take this into consideration for later ui improvements.