Different layout result when using Scene Builder (very basic) - javafx-2

Question

My first approach (which works fine)
I have two panes, gridPane (inner one) and borderPane (outer one). The grid pane is just a row of a few labels, so it is very low. It is designed with the Scene Builder. It gets loaded with the FXMLLoader and put into the center of the border pane which is created with the classical
BorderPane borderPane = new BorderPane();
The size of the window is as expected, nothing is too big or too small.
The second approach
I use Scene Builder to create the borderPane as well, but the grid pane does not get put into it right away in the Scene Builder.
FXMLLoader loader = new FXMLLoader();
loader.setLocation(Main.class.getResource("view/RootLayout.fxml"));
borderPane = (BorderPane) loader.load();
// ...
... and later on the gridPane:
borderPane.setCenter(gridPane);
When I run the app now, the window is wider and a lot higher.
Side note:
The borderPane's min/pref/max widths and heights all USE_COMPUTED_SIZE. The labels all use only USE_COMPUTED_SIZE. The ´gridPane` has 8 columns, each having min/pref width of 100. But the displayed window is > 1400 px wide and almost 800 px high. Like I said: it works fine if the border pane is created the classical way.
Here are the FXML files for both panes:
RootLayout.fxml (BorderPane, a.k.a. outer pane)
<?xml version="1.0" encoding="UTF-8"?>
<?import javafx.scene.layout.BorderPane?>
<BorderPane xmlns:fx="http://javafx.com/fxml/1">
</BorderPane>
PlacesOverview.fxml (GridPane, a.k.a. inner pane)
<?xml version="1.0" encoding="UTF-8"?>
<?import javafx.scene.control.*?>
<?import java.lang.*?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.layout.GridPane?>
<GridPane xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1">
<columnConstraints>
<ColumnConstraints minWidth="100.0" prefWidth="100.0" />
<ColumnConstraints halignment="CENTER" minWidth="100.0" prefWidth="100.0" />
<ColumnConstraints halignment="CENTER" minWidth="100.0" prefWidth="100.0" />
<ColumnConstraints halignment="CENTER" minWidth="100.0" prefWidth="100.0" />
<ColumnConstraints halignment="CENTER" minWidth="100.0" prefWidth="100.0" />
<ColumnConstraints halignment="CENTER" minWidth="100.0" prefWidth="100.0" />
<ColumnConstraints halignment="CENTER" minWidth="100.0" prefWidth="100.0" />
<ColumnConstraints halignment="CENTER" minWidth="100.0" prefWidth="100.0" />
</columnConstraints>
<rowConstraints>
<RowConstraints />
</rowConstraints>
<children>
<Label text="Name" GridPane.halignment="LEFT" />
<Label text="Montag" GridPane.columnIndex="1" GridPane.halignment="CENTER" />
<Label text="Dienstag" GridPane.columnIndex="2" GridPane.halignment="CENTER" />
<Label text="Mittwoch" GridPane.columnIndex="3" GridPane.halignment="CENTER" />
<Label text="Donnerstag" GridPane.columnIndex="4" GridPane.halignment="CENTER" />
<Label text="Freitag" GridPane.columnIndex="5" GridPane.halignment="CENTER" />
<Label text="Samstag" GridPane.columnIndex="6" GridPane.halignment="CENTER" />
<Label text="Sonntag" GridPane.columnIndex="7" GridPane.halignment="CENTER" />
</children>
</GridPane>