BUG: DataFrameGroupby.transform("size") fails
- [x] closes #27469
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry

axis=1 produces incorrect results for both aggregations and transforms. Opened #45715 and will be tackling separately.
is this a regression in 1.4.x? or just not tested previously @jreback

> is this a regression in 1.4.x? or just not tested previously

Not sure if it's a regression - if it is it's from before July 2019. thanks @rhshadrach 