With my 3 grandchildren it's worked well to buy the size that matches the age. With each one I bought clothes ahead of time in 3 mos, 6 mos, 12, mos, 18 mos, 24 mos. etc. all during the months before birth and continuing. I bought all those sized all along. For example before birth I had a few of most of those sizes. Then when they reached the size of the clothes, no matter what their age, they wore those clothes. Most of the time the age and size were close. But not always. My nearly 2 yo granddaughter is still wearing 18 mos clothes. She wore 12 mos at 18 mos but it's looking like she'll be in 24 mos soon after she's 2. she's having a growth spurt.
So, in shopping now, I buy size 3 for next winter because there isn't a 2 1/2. Last winter I bought size 18 mos for this winter. I look for clothes that will go into the next season. i.e multi season styles. Long sleeves and long pants work into mid summer here. She wasn't ready for the 18 mos size until February but she had a growth spurt and she's started into them this month. She had 12 mos clothes left over from summer to wear.
It's OK to wear clothes a bit tight and also too big. We roll up waistbands and legs often.
What clothes look like in size can be misleading. I've bought clothes that looked too big that weren't and vice versa. I just don't worry too much about the size. She can wear them when they fit. Maybe I can do this because Oregon temperatures are pretty much cool except for a couple of months in the summer and even then there are only a few days that are over 85 or so. We need jackets for evenings even in the summer.
As to the swim suit. She'll most likely be growing in length and so her butt will be close to the same size she is now. And buy a swim suit that has stretch in it to account for growth.
I suggest that you buy an assortment of sizes and then she'll grow into them when she grows into them.