The Ant Transform task (like the standard trax task on which it is based) reuses the same Transformer when performing multiple transformations using the same stylesheet. The effect of this is that source documents are retained in memory from one transformation to the next. Given the typical usage pattern of Ant, it is probably better to release source documents between transformations (if the same source document is needed again it will then be reloaded).
Note that the Ant Transform task is now released separately from the rest of the Saxon product, so this will not be fixed until a new release is available.
An update to the source code to fix this problem (not yet tested) is attached to this bug report.