See RT-10376, particularly the following comments:

"It seems setMaximized() is honored only after actual Stage rendering on screen.
This has adverse effect on restoring component sizes at startup."
https://javafx-jira.kenai.com/browse/RT-10376?focusedCommentId=354125&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-354125

"I have the same problem as Mauro Condarelli. If you create a Stage and call setMaximized() before calling show(), the Stage will NOT appear as maximized.
You have to wait until the stage is visible on screen, and then call setMaximized(). This leads to an ugly flicker on my screen if I use Platform.runLater(), because the window appears at one size, and then flickers as it resizes to the new, correct size. Would really be nice if I could make it simply appear at the correct size."
https://javafx-jira.kenai.com/browse/RT-10376?focusedCommentId=369605&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-369605