Don't worry if you think any pieces look too big or too small whilst they are not on your body. It's how they fit on you that counts. It's no good having a super nice looking thigh if you can't even get your leg it it
If the parts fit you now as they are, mark with a pencil and trim. I would strongly recommend going for the joining strips. It's the little things that make up a great looking armour. If the back has a joining strip moulded into it and you need to trim past that, then do it. It's all about getting the parts to fit right and I don't like to see excessive padding in armour just because the moulded joining strip makes the armour too big for the wearer.
Also, in your photo, you have a left thigh and a right shin shown.