To put it simply, men and women are not built the same. Being a comic and video game nerd, I find that men have a wider range of t-shirts that I like but can never find in women's sizes. I use the below trick to make my t-shirts me-sized!
Put the t-shirt on inside-out. With a fabric pen or chalk, mark off how much you will need to take in by pinching each side with the original seam on its natural fold. Do this a few times along each side so you have 5 or 6 marks on each side. You may also want to do this for the sleeves, as they're generally very loose. Take off the t-shirt and lay it on a flat surface (still inside-out). Connect the lines you drew just like in the diagram below.
Pin your shirt along those lines and sew each side. For best results, start at the bottom of the shirt and work your way up each side. When you're finished, you'll notice that the sleeves (and possibly the shirt itself) are longer. On me, these shirts have elbow-length sleeves and sit a few inches below my waist. It may be different for you, though! If you like how the shirt fits at this point, you're done! If not, you can hem the sleeves and/or the bottom of the shirt!