Perfectly normal behaviour with a bigger litter. When I first started breeding rats and a female split her litter, I "helped" her by putting them all back together. I'll tell you I got some looks from those poor female rats who I'm sure didn't appreciate the idiot human "helping" them. LOL
They'll split the litter to allow every baby to get to a teat easily and also in case a predator showed up they can't eat the whole group that way. They'll pile them back together for nightly butt washing time and so forth.
I like my females seperated to give birth, especially the new mothers, so I can see whose got good mothering skills or who might be birthing defective babies, etc. Once the litters have their eyes open at around 15 days I generally put two moms and all their babies into a big tub so the females can finish raising their young communally and get some socializing with another adult female rat (also frees up my limited single maternity tanks).