Hi Gina!
Bigger diapers are not necessarily the right solution, but a snug/not tight fit is the best. Many diapers have that second little elastic around the legs to help with such incidents. Tucking the diaper in on their back, folding it inward can help keep some of it in. That said, with your baby's sitting position, there is really only one place for that poop to go: up the back!
As a former daycare provider, I have seen my share and variety of poops! Color, size, density, consistency, and smell should be pretty predictable at this age if there are no other changes in your baby's lifestyle. Every baby and toddler has their signature poop! and when there are changes in that signature, there is usually a good reason!
My concern is it sounds pretty mooshy...this is not necessarily a sign of a healthy poop. Also, be aware of how the poop smells...if you have not changed her diet recently, and if the poop smells different, pungent, it could be she is not digesting properly and has an enzyme inbalance. Her poop should never be liquidy, even if she is breastfed.
If you are breastfeeding, have you changed your diet? Is she more gassy than usual?
Stay away from any juices or sugary (even watered down) drinks when her poops are explosive. (actually most juices are not dietarily necessary at this age anyway, and should only be used if your baby is constipated) Always better to give water if you feel your baby is dehydrated in any way. But don't use bottles for soothing or entertainment to keep her quiet. If she's on formula bottles, she shouldn't be dehydrated anyways.
Is she on formula only? have you changed formulas recently? I'm guessing she is at least on cereal by now? Do these incidents happen after eating a particular food or type of vegetable or fruit if you have her on solids?
Not all children can digest certain foods, and this can create an inbalance in digestive enzymes which can be corrected quickly with the proper diet. Your pediatrician can do a fecal test to see if there is anything to be concerned about and give you tips on how to correct her diet and restabilize her.
Been there done that,
In friendship and support,
V.