Windows Forms UserControls hosted in WPF spilling over side of window - wpf

Question

I'm converting an old WinForms app to WPF. To speed the migration, I'd like to host as many of the old app's UserControls as possible in WindowsFormsHost elements.
When I do, however, the controls seem to spill over the side of the window! It's as if the hosted control believes the window is about 20% wider than it actually is.
To illustrate what I mean, here's a shot of the WinForms control in the designer:
And here's a shot of the control once it's hosted in the app:
Has anyone else seen this before? What is this caused by, and how do I fix it?
The WindowsFormsHost element is inside a TabControl's TabItem, if that makes any difference.
Edit: Here is the relevant XML:
<Window x:Class="MyWPFApp.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:forms="clr-namespace:MyWinFormsApp;assembly=MyWinFormsApp"
Title="MainWindow" Height="600" Width="800">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition/>
<RowDefinition Height="3*"/>
</Grid.RowDefinitions>
<!-- Grid Row 1 -->
<!-- Other controls here in row 1 -->
<!-- Grid Row 2 -->
<TabControl Grid.Row="2">
<TabItem Header="Advanced">
<WindowsFormsHost>
<forms:BinaryCommandControl/>
</WindowsFormsHost>
</TabItem>
<TabItem Header="Options">
<WindowsFormsHost>
<forms:OptionsControl/>
</WindowsFormsHost>
</TabItem>
</TabControl>
</Grid>
</Window>
BinaryCommandControl and OptionsControl are both custom WinForms UserControl elements.