This is a known issue in Windows Live Mail (WLM).
When composing an email, the email signature's spacing appears larger or double spaced. See example below.
On testing, all email signatures still showed fine in our supported email clients when it was received and in the sender's sent items. The only issue is when the email is being composed.
There is no fix for this issue currently, as it is caused by the Windows Live Mail software rendering in compose mode. It does not seem to affect the quality of the signature once it has been sent.