You could probably attach yarn on the neck hole, and crochet a few rounds inside it. That would make it smaller. As far as it being too big and heavy, I'm not sure what you could do to help that. Maybe you could make a few more rectangles, and unattach the ones you have, realign them with the new ones and make a really cozy blanket with it instead. (Don't forget to crochet a border around all of your pieces once they're attached!) Then, if you're feeling ambitious, get some lighter-weight yarn and try your poncho again. Hope this helped.