关于FLEX 里控件宽度的一个问题

<mx:VBox xmlns:mx="http://www.adobe.com/2006/mxml"

xmlns:aggregatedDemond="com.sft.gec.view.trade.aggregatedDemond.*">

<mx:HBox>

<aggregatedDemond:BorrowsOverviewMcmp id="borrowsPane"/>

<aggregatedDemond:AllocationOverviewMcmp id="allocationPane"/>

</mx:HBox>

</mx:VBox>

BorrowsOverviewMcmp是

<?xml version="1.0" encoding="utf-8"?>

<mx:Canvas xmlns:mx="http://www.adobe.com/2006/mxml"

xmlns:common="com.sft.gec.component.common.*"

xmlns="com.sft.gec.view.trade.aggregatedDemond.*"

width="100%" height="100%"

verticalScrollPolicy="off" horizontalScrollPolicy="off" >

<common:NotifyIndexChangingTabNavigator width="100%" height="100%"

paddingTop="0" paddingBottom="0" paddingLeft="0" paddingRight="0"

backgroundAlpha="0" borderThickness="0" creationPolicy="all">

<TabSingleInput label="Single" />

<TabBatchInput label="Batch" />

<TabNoDemandBorrows label="No Demand Borrows" />

</common:NotifyIndexChangingTabNavigator>

<mx:HBox id="buttonBar" x="{this.width - buttonBar.width}" horizontalGap="2">

<mx:Button label="Submit" height="22"/>

<mx:Button id="upload" styleName="downloadButton" width="24" height="24"

toolTip="Upload the file"/>

<mx:Button id="paste" styleName="downloadButton" width="24" height="24"

toolTip="Paste"/>

</mx:HBox>

</mx:Canvas>

里面控件的宽度是正常的，但若删掉AllocationOverview变成

<mx:VBox xmlns:mx="http://www.adobe.com/2006/mxml"

xmlns:aggregatedDemond="com.sft.gec.view.trade.aggregatedDemond.*">

<mx:HBox>

<aggregatedDemond:BorrowsOverviewMcmp id="borrowsPane"/>

</mx:HBox>

</mx:VBox>

则宽度就变得很小了，为什么？？