Crochet rectangle in the round

[Nicolapen]

I have done 4 rounds of a rectangle starting with a chain and increasing by 3 SC in the corners. Chain 1 counts as an increase stitch. Joined rounds.

Why is one long side ending up longer than the other and the short sides curving out in an arch

[Granny Square]

You have too much bulk in the corners (not your fault, that's what happens with the typical advice to put 3 stitches to turn a the corner in SC). 

You do need 3 stitches, but it looks better if it is not 3 SCs, but rather SC, chain 1, SC.  The chain allows the 2 sides sort of 'fold' together for a sharper corner.

Similarly for a DC corner, typical advice is 5 stitches, but 2 DC, chain 1, 2 DC looks a little better.  If you do any machine sewing, it's sort of like 'grading a seam' where you trim bulk out of the seam of something that turns a corner like a shirt collar.

[Ellie 13]

I usually [sc, ch 2, sc] for the corner, then [sc, ch 2, sc] in the ch space in the next round.

Ellie 13