Winston romanticizes her appearance much as he romanticizes the potential for the proles to revolt. He sees in this woman some imaginary freedom from the spying and multitude of controls on his life, some potential for rebellion against Big Brother. Only later, when he experience the Ministry of Love, does he realize that she is not free --- that she is irrelevant as are all the proles.
Seeing her as beautiful, Winston was committing an act of rebellion, in a way. The "approved" definition of beauty, according to the Thought Police, would probably mean that this woman would not be beautiful. Yet, she was natural. She was what she was, rather than what she was t"told" to be by the society. She herself was rebelling, or so Winston thought. You might say "Ugly is Beautiful."
Winston was appreciating the obese woman, not for her physical appreance, but for her noncomformity. Interesting thought: Did the world of 1984 have supermodels? In our day and age, this is the "acceptable" appearance for a woman, which many women try to conform to. Orwell was perhaps presaging the intense focus on the "importance" of physical appearance as a means of being "acceptable."