oncheck -pT question

I altered a table columns from char(60) to varchar(100). When I did oncheck -pT on the table the extents were collapsed into 1 extent, however the first and next extent size still showed 8 when the total number of pages allocated was 160. Why is that ? Also, the amount of free space in Home Page increased 4 times. Why is that? I had expected all the data to be rearranged and packed with some percentage as free space on each page. Also, when you do an alter table does informix assign a new extent and marks the existing ones as free? Thanks Mukta