Unidentified Chinese Coin

Your coin appears to be a tong bao cash coin from the Qing Dynasty. Emperor Gao Zong ruled under the reign title Qian Long from 1736 to 1795. The coin, would have been cast at the Board of Works mint in Peking (Beijing), and should have a diameter of 26 mm and a weight of approx. 5.3 grams.
This link is an excellent one for helping to identify Imperial Chinese coins:-

http://primaltrek.com/chinesecoins....ynasty_coins

The top coin was actually cast in the Revenue mint, not the Works mint.

The difference in sizes is quite typical of genuine cash coins, especially those produced over such a long time period as the Qian Long coins were. These two were, indeed, made at different times. Dating cash coins is tricky; you need to look for fiddly little diferences in the characters, a task which can sometimes be made harder on coins that are worn or poorly cast.

On the top character on both coins, the bottom right quadrant is shaped like a letter Z; this dates both of these coin to the period prior to 1774. On the coin to the left, the character to the right has a "head" in the upper right quadrant shaped like a backwards C; this indicates it was made in the period 1761-1774. For the coin on the right, that same part of the character to the right is shaped like a sideways F, indicating earlier manufacture (1736-1760).

The smaller coin is therefore the newer of the two, which is what we would expect; cash coins generally get smaller over time.