If one of you were to tell your girlfriend who is fat that she was slim or describe her in a way that is false, it would be recorded in your book as a lie in addition to the sins of the haram relationship.

But if a man were to tell his wife, who is as fat as his mattress that she was the slimmest creature he has ever met, or describe her with the most beautiful features even if untrue, it will not be recorded as a lie because it is permissible to lie to please one's spouse.
The prophet صلى الله عليه وسلم was reported to have said:

"لا يصلح الكذب إلا في ثلاث: يحدث الرجل امرأته ليرضيها والكذب في الحرب والكذب ليصلح بين الناس"
 “Lies are not appropriate except in three cases: when a man speaks to his wife to please her, telling lies at times of war, and lying in order to bring about reconciliation between people.” 
Shaykh al-Albaani said: it is hasan. 

الراوي : أسماء بنت يزيد أم سلمة الأنصارية.
المحدث : الألباني.
المصدر : صحيح الجامع.
الصفحة أو الرقم: 7723.
خلاصة حكم المحدث : حسن

Those your haram relationships do nothing but help fill up your book of deeds with sins. Abandon them O Muslims and embrace marriage where you will be rewarded for having intercourse with your wife! What could be more beautiful? Pleasure and reward rolled in one!

A. Haroun