Bag collapses

Hi, I made a crocheted bag using T-shirt yarn (22 cm leather base) with single crochet stitches (crocheted through the back loop) and an 8 mm crochet hook.
Now I wanted to crochet the same bag with a 30 cm leather base. The bag isn’t sturdy and collapses.
What’s causing this? Does anyone have any tips?
Best regards, Christine 

With the same yarn and the same hook size? And the smaller bag stands on its own? That’s strange. I would have assumed that using T-shirt yarn would result in a softer, more flexible bag anyway.
Does it really have to be able to stand on its own?

Hi, I think you’d need to use a much smaller hook with the T-shirt yarn so it’s firm enough not to collapse at 30 cm, or you could sew in some underwire—for example, from an old bra—along the inside of the long side. I haven’t crocheted a bag myself yet, but when I sew, I just use firm or soft interfacing in the lining. 

Hi Veronika, thank you for your reply. The small bag stands up perfectly. I’m making the larger bag for a colleague. She wants the bag to stand up on its own. I’m going to try using a smaller crochet hook and single crochet stitches now. Best regards, Christine 

Thank you so much for the tip. I’ll try using a smaller crochet hook and single crochet stitches. Best regards, Christine