understanding data semantics and diagram
It seems to me that the 8763 EPROM has 16K*8 = 128kb of memory while
the 81C61 has 8kb of RAM.

The following are Program Memory Devices:
8361: 8K x 8-bit Masked Read-Only Memory(MROM) with 128x8-bit Static Random Access Memory(SRAM)
8762: 16K x 8-blt U.V. Erasable and Programmable Read-Only Memory(EPROM) with 5-bit read-only port
8763: 32K x 8-bit U.V. Erasable and Programmable Read-Only Memory(EPROM) with 5-bit read-only port

The following are Read/Write Memory Devices:
81C61: (2Kx8-Bit) RAM-I/O and 81C62 (2Kx8-Bit) RAM/CART
81C65: RAM/CART/DUCE and 81C66 RAM/EBC/DUCE
6303: (128x8-Bit) Static RAM


Are they related to the 8361 chip or are the values just a
coincidence?

The 8361 device is designed to provide non-volatile memory for program storage as well as additional working memory space for the microprocessor. It is also designed as a multiple applications device where three different 512-byte blocks of data(e.g., calibration look-uptables, constants, etc.)can reside on the device at one time.
The 8763 contain a general purpose memory-mapped 5-bit read-only input port that can be used to extend the input capabilities of a system, or as a coded input for selecting any one of several calibration tables that may be contained with in a device.
The 81C61 is designed to provide additional working memory space for the microprocessors and, in addition to providing serial I/O capabilities to the system via the custom asynchronous receiver/transmitter (CART) which is available on the 81C62 only.
Simplified Block Diagrams:

Figure D-1 8361 ROM/RAM

Figure D-2 8762/8763/87C66/87F66 EEPROM

Figure D-4 81C61 RAM-I/O and 81C62 RAM/CART

Any suggestions to further explore? (opening the microcontroller etc?)

There are lots of resources if you search 'Tuning EEC-IV' and/or '8061 Disassembly'.
The most active web resource(current as of Jan. 2021) for EEC-IV and EEC-V (8061,8065) CPUs is located at http://efidynotuning.com