I have never been to Auschwitz sites and its on my bucket list. The Birkenau camp site is indeed massive and there would be thousands of artifacts buried below a thin surface of soil which gets pushed up over time or uncovered by the elements as Harry said. I've seen it myself on the battlefields of WW1 and 200 tons of debris gets carted off annually from the fields. They expect 200 more tons, per year for the next few decades as well. Tourists typically won't venture far off the beaten path either.
Visiting in winter I imagine connects you with the suffering and sad history the inmates must have felt more than a summer visit. To think the inmates had no winter clothing or boots, were malnourished and filthy, the suffering would have been incredible. I have read many books and accounts of Auschwitz and the camps, they said the only way to survive was to get a job inside.
Have you been to the other camps at Chelmno, Majdenak, Treblinka, Sobibor and Belzec?