It is not so much that they increased so much as they discovered perspective. I am going to explain this from an art and architecture perspective so I will be a bit strange as compared to most in this sub. Late medieval art begins to have some objects viewed at the oblique but never had a mathematical explanation of perspective until Alberti came around. Alberti figured out how to do mathematically correct perspective where there were vanishing points and measurable distances. As such he was able to craft drawings that were equivalent to how we would view them in the real world. Later applications of this can be seen at the baptistry at the Duomo in Florence.

For further reading: A World History of Architecture by Michael Fazio, Marian Moffett, Lawrence Wodehouse.

A tricky question that requires a complex response, I'm on mobile now so a bit difficult but I'll try.

The biggest change from the medieval period to Renaissance period was that communities which had remained mostly isolated during the early Middle Ages began to expand their trade networks as time progressed and they connected with more and more population centers. These connections also allowed the passage of ideas as well as trade, so that artistic advances were no longer lost once the "discoverer" passed away, rather they were shared between different apprentice schools in different towns and cities. Once this sharing of ideas took hold, art was able to build upon itself over time so you see much greater advancement of artistic abilities in the Renaissance. Kind of like a snowball effect.

Between the medieval era and the Renaissance you also have a lot of developments in industries like textiles and banking, which allowed a merchant class to profit alongside nobility, and many of these chose to use the excess funds which they obtained to sponsor artists. The additional amounts of money poured into the profession allowed for artists to have studios with apprentices who could learn and later pass on the things they learned from their masters.

There are many impressive medieval artists who simply did not pass their discoveries onto others, so we erroneously think that there was very little skill during this time period.

The linking of economies, communities, and artistic schools of thought and the sharing of all these ideas led to a rapid expansion of artistic styles and an increase in realism.

I'm drivingn home from Christmas dinner with the family now (no I'm not behind the wheel) but don't have any of my books to source this at the moment but if you want to delve into this further please let me know what interests you and I can elaborate once I'm home.

Also, merry Christmas guys and girls, this is a truly wonderful sub that I'm so happy to be a part of!