Old post being closed.
There are many previous forum posts that you can read about this that give solutions.
https://www.wix.com/corvid/forum/community-discussion/inexplicable-blank-white-space-under-collapsed-strips/p-1/dl-5af45d2eadd7000030a314ee
https://www.wix.com/corvid/forum/community-discussion/feedback-empty-space-with-collapsed-elements
https://www.wix.com/corvid/forum/community-discussion/collapse-elements-unwanted-white-space
https://www.wix.com/corvid/forum/community-discussion/issues-with-collapsed-elements-blank-space-when-collapsed-container-is-not-collapsing-on-mobile