noun
bottom gusset
Refers to the gusset (extra piece of fabric) inserted at the bottom of a bag or pouch to create depth and volume.
このトートバッグは底マチが広いので、たくさん入ります。
This tote bag has a wide bottom gusset, so it can hold a lot.
底マチ付きのポーチは見た目より収納力があります。
A pouch with a bottom gusset has more storage capacity than it looks.
マチ alone means 'gusset' in general; 底マチ specifically refers to the gusset at the bottom of a bag or pouch.
Compound of 底 (そこ, 'bottom') and マチ ('gusset'), a term used in sewing and bag-making. The exact origin of マチ is uncertain, but it is commonly written in katakana in this context.