Unable to understand index only scan as it is not happening for one table while it happens for other

Please help me to understand the following,

*For the following query Index Only Scan to be performance as I am querying
the indexed fields alone. There are 20 fields in this table, inclusive of
the three I am selecting.*
localdb=# explain analyse verbose select uid, guid from mm where uid=100
limit 1;
QUERY
PLAN
------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=0.27..8.29 rows=1 width=45) (actual time=0.017..0.017 rows=1
loops=1)
Output: uid, guid
-> Index Scan using mm_pkey on public.mm (cost=0.27..8.29 rows=1
width=45) (actual time=0.016..0.016 rows=1 loops=1)
Output: uid, guid
Index Cond: (mm.uid = 100)
Planning time: 0.149 ms
Execution time: 0.042 ms
(7 rows)

*Index only scan runs for the following*
localdb=# explain analyze verbose select id, z from test order by id limit
20;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------
Limit (cost=0.28..10.29 rows=20 width=17) (actual time=0.017..0.027
rows=20 loops=1)
Output: id, z
-> Index Only Scan using zx on public.test (cost=0.28..1502.47
rows=3000 width=17) (actual time=0.016..0.024 rows=20 loops=1)
Output: id, z
Heap Fetches: 20
Planning time: 0.080 ms
Execution time: 0.050 ms
(7 rows)

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Thanks,
Rajan.
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