I don't know what you call it, but I call it the "insertion point"... this is the vertical bar which indicates the next place to place text in the textbox editor. Well, when I create a standard text box and start typing, the "insertion point" is not the size of the text I have chosen; it's maybe 3 times the size. Consequently, after either pressing the return key or the line doing a "word wrap", I have this huge space between lines.
IMO you are using a very large (or very small) font, check the style class you assigned to the textbox. Check also, like jtarin said, the fonts you are using.