Trying to mitre a finished panel but results are not great
I would agree with Isherwood and they can be tough. looking at the 2 pieces the angle of the blade is slightly off causing the difference in size.

The gauges on most home owner type table saws is a ball park, use a triangle square to set your blade angle not a small one one close to the size of your blade.
Carbide tips use a spacer at the bottom and you can get an exact 45 every time. 

Making sure your fence is square. Clamp a pice of 6” x 1” to your miter slide with the gauge at zero and the saw blade at 90 cut the board using the standard right side and verify the leading and trailing edge are exactly the same length (mark the first edge then slide forward the 6” make sure they are the same) now switch to the left slide and repeat you are looking to verify the blade is true with the table. If both sides the tip ant tail of a 6” cut are true the blade is true with the table if it is not this can cause binding and uneven cuts depending on the side of the blade used. 
The last step is to make sure your fence is true, I should have mentioned this on the first cut, if tour blade is true with the table cut a piece and make a mark at each end of the table, I actually used a sharp chisel to make these 2 marks permanent because I always use carbide blades when you set your fence measure from these points to verify the fence is parallel to the blade. This might sound like a lot to do but it is what it took to get a craftsman saw true enough to use on job sites and have results like our big saw.

When I was young my dad had a large table saw 14 or 16” for the cabinet shop this had precision guides and we still had gauge blocks to verify the blade was at the exact angle and the fence was geared at both ends so it was always square. The fence being off or the blade off a degree or so will show up on the end as you have. Use the above process and your cuts can end up exact.