The two stories do not feature the same character. The original German names for the two characters are "Daumsdick" (Literally, "Thumb-thick") for the former, and "Daumerling" for the latter.
It is Aarne-Thompson type 700, Tom Thumb.
In the first story, "Thumbling", a poor childless peasant couple wishes for a child "no matter how small" aloud. Seven months later the wife has a small child "no longer than a thumb" which they call "Thumbling" and who becomes a "wise and nimble creature." Thumbling as he grows wishes to help his father in the chores so one day asks if he can lead their horse to where his father is working by sitting in the horse's ear and giving it directions. As Thumbling performs this chore, two strange men notice the horse being led by a loud voice, and when they find out the voice belongs to a person sitting in the horse's ear, ask the peasant if they can buy Thumbling to "make a fortune" in exhibiting the little man. Thumbling convinces the peasant to take the money and leaves with the men by sitting on the brim of one of the men's hats. Then after a while Thumbling tricks the men into letting him down and he goes to hide in a mouse hole.
Later in the night Thumbling tries to sleep in a snail shell but is awakened by the sound of robbers plotting to rob a pastor's house. Thumbling yells out to them to take him along and he will help them rob it, by going into the house and handing things out to them. The robbers agree to carry him to the pastor and Thumbling makes a whole lot of noise in the house pretending to help the robbers steal. Thumbling wakes people up by yelling things like "What do you want? Do you want everything...?" making the robbery very obvious. A maid wakes up and scares off the robbers but does not see Thumbling. Thumbling gets a good night sleep in the hay. However, in the morning the maid feeds the hay that he was sleeping in to the cow. Thumbling begins to yell from the cow's stomach but the pastor thinks that an "evil spirit" had entered the cow, and has it killed. The cow's stomach is thrown into a dung heap, and before Thumbling climbs all the way out of the stomach, a wolf eats it. Thumbling, now inside of the wolf's stomach, persuades the wolf to take him home to his parents' on pretense of eating everything there. His parents kill the wolf to get Thumbling out and promise never to sell him again, not for "all the riches in the world." They give him food, drink and new clothes.
In the second story, "Thumbling's Travels" (alternately translated as "Thumbling as Journeyman"), there are similarities and differences. In this version he is a thumb-sized son of a tailor, and sometimes called a little tailor. Thumbling sets out into the world to seek his fortune. Before his departure, he is given a darning needle sword by his father and a final meal by his mother. The steam from the cooking pot carries Thumbling up the chimney and away from home. The little man goes to the house of a master craftsman, seeking to apprentice with him, but is displeased by the food there. He taunts the mistress of the house, who proceeds to chase him about the tabletop with a dishcloth seeking to strike or catch him like an insect. She eventually drives him from the house.
In the forest, Thumbling is found by a band of robbers, who recruit him help them rob the king's treasure chamber. Sneaking past the sentries, Thumbling begins to toss coins from the chamber window to the robbers. This perplexes the king and guards, as they cannot find the hidden thief but are certain they are being robbed. Thumbling mocks them as they chase the invisible intruder about the treasure chamber. He eventually rides down with the last of the coins and escapes. The robbers offer to make him their captain but he declines.
Thumbling next hires himself out as a servant at an inn but quickly annoys the maids, for he secretly watches them and reports back to the innkeeper when they steal from the food cellar. To get revenge, a maid mows him up with a patch of grass and feeds it to the cows. Thumbling is (like in the other version of the story) devoured by a cow and befuddles the innkeeper when his voice emanates from within it. The cow is slaughtered and though Thumbling tries to escape from its innards, he ends up being cooked into a black pudding with the meat. The pudding is eventually cut into slices by the mistress of the house and Thumbling once again barely escapes with his life.
Resuming his journey, Thumbling is next snapped up by a fox in the woods but manages to get himself caught in its craw. Thumbling convinces the fox to release him and rides the animal back to his father's house, done exploring the world. His father is so pleased to see him, he allows the fox to eat the chickens in the henhouse, for "he would surely love his child more than the fowls in the yard."