Alright, so here's how to make your spartan armor easier to put on, as well as potentially making it wider.
To begin with, very few people have bodies like Spartans. They have "heroic" proportions, which means that even someone as full of muscle as them won't necessarily fill out the armor the same way.
Typically what this means is that the rib circumference of the armor is bigger than necessary to fit someone of the prescribed height. This makes adjusting it to someone wider considerably easier, because it becomes simply a question of making it possible to put it on.
So let's start with what you'll NEED:
Velcro (purchasable at walmart, Jo Ann's, Home Depot, etc, you'll need 2 pieces as long as the rib section is wide, which, since most packs come 6 feet long, shouldn't be a problem)
Hot Glue gun
What you'll WANT (these will do a better job than just the above alone)
Contact/Barge cement-this will work better than hot glue, but will take time to cure into a truly unbreakable weld (though it will be functional within an hour after application). You'll also need to apply it outdoors, because the fumes are comparable to paint fumes.
Heat Knife-this will cut foam better than the knife will
Parachute Buckles-these are backup, in case the velcro can't hold the pieces alone
Dremel tool, primarily any sort of small rotary sander
Now, our primary focus in sizing:
To start, you'll need to cut on the red line here:
This should cause your chest piece to clamshell as the shoulders are the only thing attaching the back to the front, and should make getting in and out considerably easier. In order to close it up again and keep it that way, cut some of the velcro to length so that it covers both of the faces that were cut along the red line in the last step.
Once it's cut to the appropriate length (remember, measure twice, cut once), glue it into place so that the two separate faces that were cut can now rejoin as they were originally.
If that's not enough to hold the velcro in place, take the parachute buckles and glue the male and female ends to opposite faces of the clamshell. This should add an additional level of securing mechanism to the rib and hold it in place. You'll almost certainly want to secure the parachute buckles with contact cement, as it will bond foam to plastic better and provide a stronger bond
Once the glue is dry, congrats, it should be wearable. In the event it's not, however, continue on to the next step!
Step 2: Resizing the Limb Holes
In the event that the clamshell isn't enough, here's how to make the arm holes and neckhole bigger.
The arm holes are pretty simple, just take your sharp knife and cut a bigger hole, such as the green line pictured below. This may leave a ragged edge, which you can fix with the dremel sander advised in step 1. Be sure when using it not to press the sander too hard to the foam, you want multiple soft passes to make sure you get a nice, even sanding.. Once sanded, just repaint with any black paint you choose, though I use Krylon
The neck hole is slightly more complicated. If you don't mind its absence, you can simply cut off the collar, via the red line shown below. This may leave a discolored line, which can be corrected by repainting it.
However, should you want the collar to remain, the easiest thing to do is make another collar out of a thinner material. I recommend Foamies/Craft Foam/2mm foam (you can get it in sheets from Michael's or any craft store, you won't need much) but you could also make do with some heavy cardstock in a pinch.
Use the collar you just cut off as a stencil and trace its shape onto whatever your chose replacement material is with a sharpie. Then simply cut out the shape, paint it black, and glue it into place. This should give you a full inch of extra room in the neck area.
Step 3: In the Event That That Doesn't Work
As a final measure, you can make the chest piece come apart in two halves by making a second set of cuts on the red line pictured
From here, the plan is the same as the rib adjustment, cut velcro to go along the newly removed faces, then glue it into place. In the event that's not enough, use parachute buckles (you may need additional webbing to secure them in this position) to give the bond additional strength.
This should give you plenty of space, with the addition of the other adjustments, to ensure your arms, neck, and stomach can all fit in their respective places in the chest piece.