I think you just have to forget about packing lighter when you have small children. Not possible.
When we spent our one month a year in Maine when the girls were little, we'd mail a couple of boxes of books and toys to ourselves in Maine before we left D.C. That said, local libraries are a real asset. When you guys come to Maine in August, we'll introduce you to both the Brownfield Library, which was much smaller when our girls were little, and the Fryeburg Library which introduced us to, among other things, Webster and Arnold.
I spend a fair amount of time at this time of year recommending books for people to take with them on trips. One of my big favorites for people closer to Isabel's age, but Eleanor would still enjoy it, is
10 Minutes till Bedtime. I remember when your mother (my sister) discovered it -- possibly before Eleanor was born -- so I suspect you're quite familiar with it. It's by Peggy Rathmann, who wrote the wonderful wordless Good Night Gorilla. The basic plot of this one is so wackily wonderful: a boy's father tells him he has ten minutes till bedtime, and his pet hamster organizes a tour group of hamsters (ten of them wearing numbered shirts) to watch the process. Suspense grows as one wonders if the boy will be ready by deadline, and if the hamsters will be gone by the time dad shows up for the good night kiss. Each turn of the page brings the reader a minute closer, and the pictures become increasingly chaotic. For fans of Good Night Gorilla, there are many references to that book, including the gorilla and her banana. It's an I-Spy book for toddlers: there's lots to look for. And once one is reading numbers, one can identify behavior of different hamsters: #10 is always talking, #9 is always the highest on the page. One can hand it to a child strapped into a car seat, and she can get absorbed for a while.
Less inspired, but useful for toddlers, are small lift-the-flaps. My favorite in this category is
Open the Barn Door. It's little and easy to put in a parental pocket or purse. There's one flap per page, each hiding a different farm animal, but it maintains interest.
Moving on to Eleanor, you've got the advantage of chapter books: less bulk, more time-consuming. And as your pile of books attests, paperback picture books don't take up too much space. About a half dozen of Robert Munsch's books (best known for Paper Bag Princess: we've written about him here and here) have been issued in tiny paperbacks: about three and a half inches square, but readable, with full text. Another put-in-parent's-pocket size.
Enjoy your vacation, and I look forward to seeing you soon.